Hi WoodyZ,
Thanx for the "tip" but it didn't make completely the "trick" 
When turning the 3D accelerate option off, then the sign in dialog shows up and I can start using Fedora 18.
However, I'm locked with a 800x600 screen resolution and the change of display resolution using the Fedora display option in the system setting dialog is refused and worst... freeze Fedora.
Further, if I turn the 3D accelerate option back to on, the previous problem is back.
Meanwhile, I've to redo a complete install to verify this point since I didn't used the screen host setting when I installed Fedora 18 (I selected 1 monitor 1024 x 768 that BTW isn't taken care).
I'll keep you posted.ASAP.