I do have gnome-session actually, but I get the same behavior out of a SteamOS login. https://github.com/ValveSoftware/SteamOS might be the place this goes.

(P.S. I remembered that CTL+ALT+F2, loging in, and then $ sudo killall Xorg also restarts Xorg and returns to the DM or default session unless unset in one of the config files)

I tried something similar (steamos kernel, nvidia-glx from steam repo's, added to a fresh Debian wheezy install) and found two bugs and a working-as-intended-but-I-didn't-like-it.

Bug #1:) If SteamOS is selected at session login, the only way to return to the login prompt and thus a Gnome session is to reboot. "Exit to Desktop" doesn't work.

Bug#2:) There are some kernel modules named open-vm-* that are rebuilt on boot, every reboot. The build is backgrounded to tty1 so it doesn't prevent access to login. Do these modules need to be rebuilt on every boot?

Feature #3:) modeswitch-inhibitor causes the Gnome UI and such to be sized really big.

The way I fixed #2 and #3 for now is to remove steamos-base-files and steamos-modeswitch-inhibitor.

For #1 I just don't log into a SteamOS session.

Spy sapping pyro,

Sharks' response would also work for you, although it might be -t testing or -t jessie rather than -t wheezy.

Thanks.

Coo. How did you fix the "pkgname has no installation candidate" error?

Perhaps you were looking for this or something like it?

http://www.reddit.com/r/SteamOS/comments/1t46xp/guide_run_any_nonsteam_application_such_as_xbmc/

Did you run:

$ sudo apt-get update

after adding the wheezy repo's?

Interestingly: I just found out that if one had already set up apt-pinning for accessing the Debian testing repo's on a machine set to primarily use the stable repo's, the a=testing section of the definition would include the steam repo's.

Whoops! My mistake. *edited.

+1 Hitsuji's recommendation.

Pin the Debian repo above the SteamOS repo for the purpose of that piece of software. You can use the -t modifier in the console, a la:

# apt-get -t wheezy install libxml2

This may help:

http://jaqque.sbih.org/kplug/apt-pinning.html

Yes. Make a text file, put it into the /etc/apt/preferences.d/ folder, save it as preferences

$ gksu gedit /etc/apt/preferences.d/preferences

then type;
then save as. . . etc.

gksu is like sudo or su -c; except it escalates privledges for graphical programs rather than console ones. Use sparingly, and never just because someone on the internet recommends it.

EDIT :
SteamOS does not include gksu. It would be much more appropriate to use sudo nano /etc/apt/preferences.d/preferences in the console command listed above.