I just update to 12.04 from 11.10 and I get to the login screen w/o problems. When I enter my password the screen turns black and returns to the login screen half a second later.
There is a related question out there which was solved by creating more space on the disk but my disks (or rather partitions) are all below 30%.
I can log in as guest. I can also login at the cmd prompt (going to tty with Alt+Ctrl+F1) with my normal user credentials. When logged in as guest I can also install programs using my normal account password.
There is the normal authentication error when I mistype my password so I'm also sure the password works.