After upgrading to 12.04LTS we found autologin no longer worked. The secret was to add:
autologin-user=username
autologin-user-timeout=20
to /etc/lightdm/lightdm.conf.
Refer to the post at http://www.mattfischer.com/blog/?p=343 for more info. Or look in /usr/share/doc/lightdm/lightdm.conf.gz.