My system crashed during an upgrade to Natty Narwhal. It refused to try to fix that, but instead wanted to go straight to Oneiric Ocelot. I crossed my fingers, having little other choice. Amazingly, it seemed to do the full upgrade. But then I found that I had this thread's issue.
I tried many of the suggested owner:group and permission fixes, but none had any effect. I couldn't get into Gnome, but I had SSH access, so I could make changes from there. The final fix for me was:
Then reboot (it went to lxde), and log in to Gnome to verify that it was working. Then because I didn't need it:
sudo apt-get install lxde
That's all, and everything is fine now.
sudo apt-get autoremove lxde