And the user is in groups:
disk lp wheel video storage users

How do you start KDE? Is dbus running at the boot?

Yes. B-bus is started before kde.

I use a multiseat setup. XBMC running on the other display; and XBMC able to mount the cd drive if I start with xbmc --standalone.

@farkaslory: you should not need to be in disk/lp/video/storage groups (but I don't think it will do any harm). udisks should let you mount / automount without any special permissions. Usually some permissions are granted by consolekit, is this running? It might be that consolekit is getting itself confused if you have two active users (which I guess you do if you have a multiseat setup).

Could you attach your xorg.conf file(s), so we could have a look at how your multiseat is set up?

I won't attach the xorg.conf file. Because it has no use (yes it is a complete multiseat setup with two user). Who login first is the only relevant thing.

The problem is the following: the user (called xbmc-user) which runs the xbmc log in first (because I use pulseaudio and xbmc-user start it). Then the second user with my normal login cannot mount any media.

The problem definitely with the multiple user setup (and not e.g. with xbmc). If the first user don't start xbmc, but start an another wm (I used windowmaker) the the problem is the same. If my normal login log in first, then my normal login can mount any media, but if I login with xbmc-user as the second user then xbmc cannot mount anything.

I think a temporary solution if kdm start the pulseaudio server and set the PULSE_SERVER enviroment for xbmc to attach that server (so not xbmc-user will be the first user who logs in).

But this is a bug. Udev and consolekit should work with more than one user logged in.
Has anyone any idea?

Could you post the output of
# ck-list-sessions
after logging in both users?

$ ck-list-sessions
Session4:
unix-user = '1000'
realname = 'Lorant Farkas'
seat = 'Seat1'
session-type = ''
active = FALSE
x11-display = ':0'
x11-display-device = '/dev/tty8'
display-device = ''
remote-host-name = ''
is-local = TRUE
on-since = '2011-08-31T19:50:03.165447Z'
login-session-id = '4294967295'
Session1:
unix-user = '1003'
realname = 'XBMC user'
seat = 'Seat1'
session-type = ''
active = FALSE
x11-display = ''
x11-display-device = ''
display-device = '/dev/tty1'
remote-host-name = ''
is-local = TRUE
on-since = '2011-08-31T19:49:12.603486Z'
login-session-id = '1'
idle-since-hint = '2011-08-31T19:49:43.153517Z'
Session2:
unix-user = '1003'
realname = 'XBMC user'
seat = 'Seat1'
session-type = ''
active = TRUE
x11-display = ':1'
x11-display-device = '/dev/tty7'
display-device = '/dev/tty1'
remote-host-name = ''
is-local = TRUE
on-since = '2011-08-31T19:49:17.092264Z'
login-session-id = '1'
Session5:
unix-user = '1000'
realname = 'Lorant Farkas'
seat = 'Seat3'
session-type = ''
active = FALSE
x11-display = ':0'
x11-display-device = ''
display-device = ''
remote-host-name = ''
is-local = TRUE
on-since = '2011-08-31T19:50:03.325718Z'
login-session-id = '4294967295'

I do not understand.
What is the meaning of
"x11-display-device = '/dev/tty8'"
kdm (as well as xbmc) start on VT 7.
I use the command

startx -- /usr/bin/Xorg :1 vt7 -layout layoutname2

to start the X server for xbmc-user, and

ServerArgsLocal=vt7 -nolisten tcp -layout layoutname1

in the kdmrc.

Regarding /dev/tty8: you cannot have two sessions on the same tty, so the second you try to start will just use the next available tty.

I don't know the details of this is supposed to work (never tried multiseat myself), but I think the problem might be that only one of your sessions are marked as "active". This might be related to the fact that all your sessions appear to be on Seat1, I would assume that two sessions cannot both be active if they are on the same seat, but I might be wrong on this...

I think you have to report this upstream