Just upgraded to Ubuntu 12.04 LTS (Precise Pangolin), and it was working somewhat until after a few reboots it would not allow me to login any further from lightdm.
A quick look in /var/log/auth.log showed that I was receiving the error:
lightdm: pam_succeed_if(lightdm:auth): requirement "user ingroup nopasswdlogin" not met by user "XXX"
( Where XXX is my username )
Turns out the reason for this was that somehow my ~/.Xauthority file had its ownership changed to root:root ( dont ask me how )
A quick:
sudo chown XXX:XXX ~/.Xauthority
( Where XXX is my username )
And I was able to login again!
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