My F10 xorg also locks up.
I've just upgraded to the brand new package xorg-x11-drv-ati-6.9.0-61.fc10.i386 and it's worse
It still locks up (with or without nomodeset) and now there is a bug in the font display.
I'm bored of it, and gave up using the radeon xorg driver. I'm back to the old vesa driver, and for now no font display issue and not locked up (yet?).
By the way, my radeon is a X300.
I have another radeon which does not locks up X, but it also has the font display issue.