I suffer from, https://bugs.launchpad.net/ubuntu/+source/meta-gnome2/+bug/151544, also. I don't know if it has anything to do with it. A few people in the forums who are also experiencing the same problem have confirmed this behavior on their rigs too. When you log out, you are brought to the gdm login screen, that works fine. But, when you enter the credentials, and are authenticated, the login proccess begins, but no session is created. It sits without a desktop, no wallpaper, no gnome-panel, just a mouse that you can move around. i let it sit for an hour like that, and nothing happened. It happens if you logout, or restart the desktop manager (Using /etc/init.d/gdm restart or ctrl+alt+backspace). Requiers a a restart from the virtual terminal, because restarting it again, results in the same behavior. So if you want a gui, you have to completley restart.
Thank you for reporting this bug.
You may want to try switching to a virtual console (ctrl-alt-f1), logging in there and then using "top" to see if there is some process hogging the CPU.
I'm marking this bug as duplicate, please refer to bug 151544 and see if you can provide any information there to fix the problem.