I have bumblebee working (tested on glsxpheres), steam client works (playing on iGPU client and games run-but slow).
Running $optirun steam --client works ok, but any game fails:

in dmesg i get:
vgaarb: device changed decodes: PCI:0000:01:00.0,olddecodes=none,decodes=none:owns=none
[ 6772.061509] NVRM: loading NVIDIA UNIX x86_64 Kernel Module 313.30 Wed Mar 27 16:56:45 PDT 2013
[ 6772.564518] ACPI Error: Field [TMPB] at 274432 exceeds Buffer [ROM1] size 262144 (bits) (20121018/dsopcode-236)
[ 6772.564539] ACPI Error: Method parse/execution failed [\_SB_.PCI0.PEG0.PEGP._ROM] (Node ffff880418c85f50), AE_AML_BUFFER_LIMIT (20121018/psparse-537)
[ 6892.857247] bbswitch: disabling discrete graphics
[ 6892.867698] pci 0000:01:00.0: Refused to change power state, currently in D0
[ 6892.884467] pci 0000:01:00.0: power state changed by ACPI to D3cold

which seems like ACPI/BIOS/firmware problem for my Gigabyte U2442V notebook with GT640M GPU. (Linux 3.8.5) runs fine here, I have acpi error messages, which Gigabyte responded to my BIOS fix request that they dont support Linux. So if actually anybody can reproduce the steam ACPI bug in Windows, I could report and demand a fix (is there dmesg equivalent in W8?:P)

$ dmesg | grep Error
//I used to have, and reported these:
[ 3.756723] ACPI Error: [DSSP] Namespace lookup failure, AE_NOT_FOUND (20121018/psargs-359)
[ 3.756731] ACPI Error: Method parse/execution failed [\_SB_.PCI0.SAT0.SPT1._GTF] (Node ffff880418c83140), AE_NOT_FOUND (20121018/psparse-537)
[ 3.757020] ACPI Error: [DSSP] Namespace lookup failure, AE_NOT_FOUND (20121018/psargs-359)
[ 3.757027] ACPI Error: Method parse/execution failed [\_SB_.PCI0.SAT0.SPT0._GTF] (Node ffff880418c830c8), AE_NOT_FOUND (20121018/psparse-537)
[ 3.757329] ACPI Error: [DSSP] Namespace lookup failure, AE_NOT_FOUND (20121018/psargs-359)
[ 3.757337] ACPI Error: Method parse/execution failed [\_SB_.PCI0.SAT0.SPT1._GTF] (Node ffff880418c83140), AE_NOT_FOUND (20121018/psparse-537)
[ 3.787784] ACPI Error: [DSSP] Namespace lookup failure, AE_NOT_FOUND (20121018/psargs-359)
[ 3.787792] ACPI Error: Method parse/execution failed [\_SB_.PCI0.SAT0.SPT0._GTF] (Node ffff880418c830c8), AE_NOT_FOUND (20121018/psparse-537)
[ 27.073802] ACPI Error: [DSSP] Namespace lookup failure, AE_NOT_FOUND (20121018/psargs-359)
[ 27.073812] ACPI Error: Method parse/execution failed [\_SB_.PCI0.SAT0.SPT0._GTF] (Node ffff880418c830c8), AE_NOT_FOUND (20121018/psparse-537)
[ 27.075508] ACPI Error: [DSSP] Namespace lookup failure, AE_NOT_FOUND (20121018/psargs-359)
[ 27.075516] ACPI Error: Method parse/execution failed [\_SB_.PCI0.SAT0.SPT0._GTF] (Node ffff880418c830c8), AE_NOT_FOUND (20121018/psparse-537)


//these are new, most likely related to me running a game via steam:

[ 1632.426732] ACPI Error: Field [TMPB] at 274432 exceeds Buffer [ROM1] size 262144 (bits) (20121018/dsopcode-236)
[ 1632.426751] ACPI Error: Method parse/execution failed [\_SB_.PCI0.PEG0.PEGP._ROM] (Node ffff880418c85f50), AE_AML_BUFFER_LIMIT (20121018/psparse-537)
[ 1659.948862] ACPI Error: Field [TMPB] at 274432 exceeds Buffer [ROM1] size 262144 (bits) (20121018/dsopcode-236)
[ 1659.948881] ACPI Error: Method parse/execution failed [\_SB_.PCI0.PEG0.PEGP._ROM] (Node ffff880418c85f50), AE_AML_BUFFER_LIMIT (20121018/psparse-537)
[ 1779.190316] ACPI Error: Field [TMPB] at 274432 exceeds Buffer [ROM1] size 262144 (bits) (20121018/dsopcode-236)


$ optirun steam --debug
Running Steam on arch 64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(1364601080_client)
Installing breakpad exception handler for appid(steam)/version(1364601080_client)
unlinked 0 orphaned pipes
Installing breakpad exception handler for appid(steam)/version(1364601080_client)
[0404/160753:ERROR:object_proxy.cc(239)] Failed to call method: org.freedesktop.systemd1.LoadFailed: Unit dbus-org.freedesktop.NetworkManager.service failed to load: No such file or directory. See system logs and 'systemctl status dbus-org.freedesktop.NetworkManager.service' for details.
[0404/160753:WARNING:proxy_service.cc(646)] PAC support disabled because there is no system implementation
Installing breakpad exception handler for appid(steam)/version(1364601080_client)
Installing breakpad exception handler for appid(steam)/version(1364601080_client)
Installing breakpad exception handler for appid(steam)/version(1364601080_client)
PulseAudio connect failed (used only for Mic Volume Control) with error: Access denied
Installing breakpad exception handler for appid(steam)/version(1364601080_client)
Generating new string page texture 2: 48x256, total string texture memory is 49.15 KB
Generating new string page texture 3: 256x256, total string texture memory is 311.30 KB
Installing breakpad exception handler for appid(steam)/version(1364601080_client)
Installing breakpad exception handler for appid(steam)/version(1364601080_client)
Adding license for package 0
Adding license for package 4991
Adding license for package 6896
Adding license for package 7368
Adding license for package 11793
Adding license for package 12361
Adding license for package 13602
Adding license for package 14633
Adding license for package 14938
Adding license for package 15638
Adding license for package 16020
Adding license for package 16342
Adding license for package 16360
Adding license for package 16632
Adding license for package 18207
Adding license for package 18629
Adding license for package 18773
roaming config store loaded successfully - 1270 bytes.
migrating temporary roaming config store
ExecCommandLine: "/mnt/store/steam/ubuntu12_32/steam --debug"
Generating new string page texture 70: 1024x256, total string texture memory is 1.36 MB
Generating new string page texture 71: 128x256, total string texture memory is 131.07 KB
Generating new string page texture 72: 128x256, total string texture memory is 1.49 MB
Generating new string page texture 73: 32x256, total string texture memory is 1.52 MB
Installing breakpad exception handler for appid(steam)/version(1364601080_client)
Generating new string page texture 78: 64x256, total string texture memory is 1.59 MB
Generating new string page texture 79: 128x256, total string texture memory is 1.72 MB
Generating new string page texture 80: 384x256, total string texture memory is 2.11 MB
Generating new string page texture 82: 512x256, total string texture memory is 2.64 MB
System startup time: 9.43 seconds
Generating new string page texture 91: 24x256, total string texture memory is 2.66 MB
Running Steam on arch 64-bit
STEAM_RUNTIME has been set by the user to: /mnt/store/steam/ubuntu12_32/steam-runtime
ExecCommandLine: "/home/marek/.steam/root/ubuntu12_32/steam steam://open/driverhelperready";
ExecSteamURL: "steam://open/driverhelperready"
Generating new string page texture 93: 128x256, total string texture memory is 2.79 MB
Failed to load installscript /mnt/store/steam/SteamApps/common/Osmos/installscript.vdf
Game update: AppID 29180 "Osmos", ProcID 11083, IP 0.0.0.0:0
ERROR: ld.so: object 'gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded: ignored.
ERROR: ld.so: object 'gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded: ignored.
----------------------------------------------------------------------------------------------------
Log opened on Thu Apr 4 16:08:30 2013
Commandline: ./Osmos.bin32
Preinitializing game: STEAM version 1.6.1 2302
Localization: using language "en"
Localization: loaded Osmos-en.loc
Arch: Intel(R) Core(TM) i7-3517U CPU @ 1.90GHz
OS: Linux 3.8.5-1-ARCH (#1 SMP PREEMPT Fri Mar 29 19:18:14 CET 2013)
Using sound
Showing splash
Using fullscreen mode: 1600 x 900
Not using vsync
Initializing GLRenderDevice...
OpenGL version: 4.3.0 NVIDIA 313.30
Initializing game
Initializing GLRenderer...
Loading textures
Loading fonts
Backed up stats to Stats/Backup/Osmos_0003.sta
Game removed: AppID 29180 "Osmos", ProcID 11084

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.

(steam:10984): LIBDBUSMENU-GLIB-WARNING **: Trying to remove a child that doesn't believe we're it's parent.
saving roaming config store to 'sharedconfig.vdf'
roaming config store 2 saved successfully