Just install 12.04-64bit and while I can login using the guest login, I cannot with my user login: it just cycles back to the login screen. I have performed the sequence of apt-get update/upgrade and install of the nvidia-current driver, but got back that it was already in use. The password appears to be recognized, as wrong one results in an error. Have also tried switching to the 2D Unity, without other results.
This can be the solution: http://www.icpep.org/solved-ubuntu-wont-login/
Initially from my clean install of 12.04 (64bit), the login screen would intermittently repeat, and the second time the login would be stuck with "Logging in..." under the user name.
Found a bug report about the "Logging in..." stating that this was caused by using a non-default wallpaper and an issue with lightdm. I have reverted to a plain blue background, and I now no longer get the "Logging in..." freeze issue - however, I consistently get two login screens!
I enter credentials correctly and am immediately presented with a second login screen (no errors). Authenticating against the second screen completes a normal login to the Ubuntu desktop.
Any ideas what is going on here? I don't see any obvious errors in the logs (maybe I am looking in the wrong logs).