After upgrading to 12.04.2 LTS I had the famous login issue with the dark screen showing for a split second, then going back to lightdm.
I found out I could remove ~/.Xauthority or chown/chgrp .Xauthority to my username and group to make everything work again, having to use one of the tty s to do so.
However, the issue is that I have to do this at every boot to be able to log in.
I've got an encrypted home folder which is decrypted as I log in.
I had a search first, but couldn't find a solution to my particular variant of this problem.