Since you can see lightdm I think this is more a lightdm and no plymouth problem.
A bunch of people have similar problems like you. Maybe one of these links can help you:
https://bugs.launchpad.net/lightdm/+bug/986967 (Launchpad Bug which sounds like your (and mine) problem)
similar questions here on askubuntu:
Ubuntu 12.04 Hangs On Login
Stuck at login screen (with a detailed answer on how to switch to gdm as workaround)
12.04: LightDM freezes while "Logging in..." - maybe due to deactivated login sound
edit: There is a bugfix on it's way which is already available in the precise-proposed repository. You can read here how to use it.
edit: People in Launchpad report that the bugfix is working, and that the problem may be caused by deactivating the login sound with Ubuntu Tweak. Maybe reactivating the login sound might fix this problem too until a official update is available