I'm posting this in case it might help someone else.
I changed my resolution from 1280x1024 to 1024x768 and my screen went black and didn't come back.
I had no /etc/X11/xorg.conf so I downloaded system-config-display. Changing resolutions with that tool had no effect, I still got the black screen on boot after attempting to login. I changed over to the vesa driver and that allowed me back into x. Changing back to the Radeon driver produced the black screen again. This was with no xorg.conf and with one as well.
Changing back to the Radeon driver with a reboot and adding nomodeset to the kernel line of grub worked.
This also stopped a vertical scroll issue I was having with a MS wireless mouse.
Hope that helps if you have this issue.
Edit: I resintalled to see if I could replicate this issue and it happened the same way.
Both times also when I click on "Release Notes" in Firefox http://docs.fedoraproject.org/release-notes/
the screen blanks tries to sync and kicks me out to the login screen. When I log back it firefox says a new addon has been installed but it hasn't. Other links seem to work fine and this link works fine before the video problem.
Anyone know how to fix this? The "nomodeset" alllows me to work but something is still broken just from changing the resolution.