I have been trying to upgrade a box from FC10 to FC11. I had several problems/gripes with the FC11 installer, but I will edit those at another time. This was a real drop dead problem for me.
First, I tried running the upgrade selection (instead of the full installation). Boot up seemed fine, until the gdm tried to start. Then I got garbage on the screen, and the gdm froze. I could only press the power button to shutdown. But on power up, the same thing.
So I decided to just do a full install. Everything worked fine. I could log in to Gnome, and make the typical settings changes that I do. One of those was to change the display type from the generic monitor to my mine (Viewsonic GS790). I also have a Radeon ATI Rage video card (not sure of the exact model - still looking it up). I have had no problems installing previous FC versions (9, 10) with this setup. But when I logged out to restart the gdm, I got the same problem as above - a frozen display. (I did upgrade another system that went fine - but it didn't have a Radeon card.)
I switch cards (to a non-Radeon) and voila everything works fine.
So what did they do?