"Alt-Ctrl-F2" , "Login" by Textmode. "sudo rm /tmp/.X0-lock" , "startx" then it starts up OK, after that I "Enabled Automatic Login", under "System-->Administration-->Login Window-->Safety..."
This can be a solution to this bug:
Binary package hint: gdm
I have previously installed Ubuntu 8.04beta On a Kingston Traveller 8GB stick, with success. After upgrade the screen go blank after Login from GDM. Then i tried to make a fresh installation with Ubuntu 8.04 RC and 8.04 (from yesterday) with the same result...