Description:
This bug happens when Arch is installed on an Intel Rapid Storage (Intel Matrix) RAID array, managed with mdadm. mdadm -As assembles the Intel Matrix container, its arrays and partitions correctly, with the procedure described at http://bertrandbenoit.blogspot.it/2011/07/manage-intel-raid-under-gnulinux-using.html
The only difference from normal Linux soft RAID is passing -e imsm to mdadm so it reads Intel Matrix metadata. This all works fine in a booted environment, but there seems to be no way to assemble the arrays correctly at boot.

Placing the mdadm hook in the HOOKS array in mkinitcpio.conf always leads to a segfault in mdassemble, so all I could try were various combinations of mdadm_udev in HOOKS. None work. In all cases, in the initrd, right after running early hook udev and then uevents, I get a message about /dev/md/RAID0SYS_0p5 (my root partition, tried with /dev/md125p5 and UUID as well) not existing and I'm thrown into the initrd shell. At that time, some of the md devices exist in /dev (in my case only the imsm0 device, which represents the Intel Matrix container, as mdadm sees it). The partitions do not exist anywhere in /dev. I just run mdadm -As (--assemble --scan) and everything is assembled correctly, then I can just exit and boot resumes correctly. No other manual intervention except mdadm -As is needed during boot in the initrd, with the /etc/mdadm.conf as reported below.

As a horrible temporary workaround, I have added the line mdadm -As in /usr/lib/initcpio/init_functions at the beginning of resolve_device(). This at least allows me to boot without manual intervention in the initrd shell every time, but it's obviously not a solution.

Additional info:
* package version(s):
core/linux 3.9.7-1 (base)
core/mkinitcpio 0.14.0-1
core/mdadm 3.2.6-4 (base)

* config and/or log files etc.
% cat /etc/mdadm.conf
ARRAY /dev/md/imsm0 metadata=imsm UUID=8e383f71:53f6e9a6:c92989ef:95f190d1
ARRAY /dev/md/RAID0RST_0 container=/dev/md/imsm0 member=1 UUID=bd2f77ef:7b830080:a86d22f7:b2d5dd1f
ARRAY /dev/md/RAID0SYS_0 container=/dev/md/imsm0 member=0 UUID=aceff348:bdd4eece:0059eec4:e261b3c8

% la /dev/md
total 0
drwxr-xr-x 2 root root 240 28/06/2013 10:08 ./
drwxr-xr-x 19 root root 3200 28/06/2013 10:08 ../
lrwxrwxrwx 1 root root 8 28/06/2013 10:08 imsm0 -> ../md127
lrwxrwxrwx 1 root root 8 28/06/2013 10:08 RAID0RST_0 -> ../md126
lrwxrwxrwx 1 root root 8 28/06/2013 10:08 RAID0SYS_0 -> ../md125
lrwxrwxrwx 1 root root 10 28/06/2013 10:08 RAID0SYS_0p1 -> ../md125p1
lrwxrwxrwx 1 root root 10 28/06/2013 10:08 RAID0SYS_0p2 -> ../md125p2
lrwxrwxrwx 1 root root 10 28/06/2013 10:08 RAID0SYS_0p3 -> ../md125p3
lrwxrwxrwx 1 root root 10 28/06/2013 10:08 RAID0SYS_0p4 -> ../md125p4
lrwxrwxrwx 1 root root 10 28/06/2013 10:08 RAID0SYS_0p5 -> ../md125p5
lrwxrwxrwx 1 root root 10 28/06/2013 10:08 RAID0SYS_0p6 -> ../md125p6
lrwxrwxrwx 1 root root 10 28/06/2013 10:08 RAID0SYS_0p7 -> ../md125p7

The last 3 are in order: root, home, swap. Boot is from UEFI with rEFInd on p1.

Kernel line from rEFInd: options "ro root=/dev/md/RAID0SYS_0p5 rootfstype=ext4 acpi_osi=Linux systemd.unit=graphical.target resume=/dev/md/RAID0SYS_0p7"
Tried with "domdadm" too as suggested by random people on the internet, no effect.

I also tried with different modules in mkinitcpio.conf, no luck.
% cat /etc/mkinitcpio.conf
# vim:set ft=sh
# MODULES
# The following modules are loaded before any boot hooks are
# run. Advanced users may wish to specify all system modules
# in this array. For instance:
# MODULES="piix ide_disk reiserfs"
MODULES="md_mod ext4 raid0 asus-nb-wmi"

# BINARIES
# This setting includes any additional binaries a given user may
# wish into the CPIO image. This is run last, so it may be used to
# override the actual binaries included by a given hook
# BINARIES are dependency parsed, so you may safely ignore libraries
BINARIES="/sbin/mdmon"

# FILES
# This setting is similar to BINARIES above, however, files are added
# as-is and are not parsed in any way. This is useful for config files.
FILES=""

# HOOKS
# This is the most important setting in this file. The HOOKS control the
# modules and scripts added to the image, and what happens at boot time.
# Order is important, and it is recommended that you do not change the
# order in which HOOKS are added. Run 'mkinitcpio -H <hook name>' for
# help on a given hook.
# 'base' is _required_ unless you know precisely what you are doing.
# 'udev' is _required_ in order to automatically load modules
# 'filesystems' is _required_ unless you specify your fs modules in MODULES
# Examples:
## This setup specifies all modules in the MODULES setting above.
## No raid, lvm2, or encrypted root is needed.
# HOOKS="base"
#
## This setup will autodetect all modules for your system and should
## work as a sane default
# HOOKS="base udev autodetect block filesystems"
#
## This setup will generate a 'full' image which supports most systems.
## No autodetection is done.
# HOOKS="base udev block filesystems"
#
## This setup assembles a pata mdadm array with an encrypted root FS.
## Note: See 'mkinitcpio -H mdadm' for more information on raid devices.
# HOOKS="base udev block mdadm encrypt filesystems"
#
## This setup loads an lvm2 volume group on a usb device.
# HOOKS="base udev block lvm2 filesystems"
#
## NOTE: If you have /usr on a separate partition, you MUST include the
# usr, fsck and shutdown hooks.
#HOOKS="base udev mdadm_udev autodetect modconf block filesystems keyboard fsck shutdown"
HOOKS="base udev autodetect modconf block mdadm_udev resume filesystems keyboard fsck shutdown"


# COMPRESSION
# Use this to compress the initramfs image. By default, gzip compression
# is used. Use 'cat' to create an uncompressed image.
COMPRESSION="gzip"
#COMPRESSION="bzip2"
#COMPRESSION="lzma"
#COMPRESSION="xz"
#COMPRESSION="lzop"

# COMPRESSION_OPTIONS
# Additional options for the compressor
#COMPRESSION_OPTIONS=""


Steps to reproduce:
1) Install Arch on Intel Matrix Raid/Intel Rapid Storage.
2) Configure RAID as above.
3) Can't boot from partition on RAID array unless manually executing mdadm -As in initrd shell.

From what I gather, some people on other distros have also had trouble booting from imsm.

I realize this is uncomfortable to test. I'm available as teo- on Freenode for testing, debugging and providing any other info you may need.