after updating to latest release of Xserver to:
xorg-x11-server-Xorg-1.10.2-1. fc15.i686
I got a non-working system with a blank screen at login.
System could start with nomodeset option on boot line. Using an Intel card...(it is a recursive bug in F13, F14 and now F15, that is very annoying because it is recursive, something must be done to avoid these noise.)
If I revert to previous release everything is o.k.