Comment 3 for bug 210538

This appears to be related to the GDM face browser / graphical greeter. Simon, were you using the face browser?

Lots of discussion here:
http://ubuntuforums.org/showthread.php?t=742511

Workaround is to go into console, manually reconfigure GDM to use the standard greeter.

1.Press Ctrl-Alt-F1

2. Log in as an administrative user (someone who can use sudo)

3. Edit gdm.conf eg: sudo nano /etc/gdm/gdm.conf

4. Search for: Greeter=/usr/lib/gdm/gdmgreeter
and change this to: Greeter=/usr/lib/gdm/gdmlogin