I recently ran into the same issue. In my case I have a single nvidia video card with two outputs connected to two monitors. My prefered way to set it up is so that they are two separate displays :0.0 and :0.1. This worked great in debian 6 and gnome 2.x. Each screen would get its own gnome panels, and its own separate virtual desktops that could be switched independantly.
In wheezy and gnome 3 this no longer works. You are right in that its not related to gdm, but rather the gnome shell. You should see a message like this in your ~/.xsession-errors: gnome-session-is-accelerated: Zaphod mode not supported. So basically you get the gnome-fallback session, and rather than starting a gnome panel on each display as expected, it starts two of everything on just one screen (as described in the original bug report). It starts nothing on the second screen. The workaround is to switch to xinerama/twinview mode instead which extends a single display :0 to both monitors. However it is a regression since a configuration that used to work (and is by no means exotic) is no longer supported. Infact this used to be the only way to work with multiple monitors until the advent of xinerama in XFree86 4.0. Thanks, Norbert -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org