Today i updated my Ubuntu to the new 11.10 Oneiric… And i spent 3 hours to get it work again.
The problem is, that the update is buggy. At least it was on my rig. But reading the forums, there were plenty of other people having the exactly same problem: You can’t go past the login screen. Or you can’t go past the startup screen.
The source of the problem is, that Ubuntu switched from gdm to lightdm, but somehow they forgot to completely get rid of gdm. And some other tiny problems.
After hours of struggle, i’ve found this: http://askubuntu.com/questions/66482/cant-login-after-installing-11-10.
It solved my problem