My problem is solved by installing:
lightdm-gtk-greeter

It seems that version 11.10 replace gdm by lightdm start manager and because the version upgrade didn't install the greeter package, the new lightdm didn't work.

Four hours lost due to this silly issue

This worked perfectly for me. Thanks.

Jon

worked for me too,
go to Ubuntu Software Centre > search ''lightdm'' > more info > add-ons > install "unity-greeter".
Problem solved
Thanks