FYI... revised:
https://bugzilla.redhat.com/show_bug.cgi?id=472634
after KDE 4.2 push yesterday. Previously with 4.1 Desktop effects
enabled appeared to work fine with nomodeset, but after 4.2 things
went south in a big way. Didn't matter if I used modeset or
nomodeset, X would loop. Most times I could SSH in and reboot the
system, but sometimes system completely locked up. Initially I had
opened against Radeon 200M, but now my desktop system is affected
(Radeon X300). Another report was with a Radeon X550 - so this seems
pretty pervasive with the Radeon chipset. Not sure what changed in
4.2 with desktop effects, but whatever was uncovered, triggered
badness. At this point it looks like KDE4.2 + Desktop Effects +
Radeon = Xorg Loop