Something that has really irritated me over the last few days was in Debian squeeze I was unable to get a login dialog despite the greeter background being visible. What made this worse was if I did several reboots and played around with VMware autofits... it would sometimes work!

In the end I just enabled lightdm

aptitude install lightdm

See these links for more info

https://wiki.debian.org/LightDM
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=724731