Just select recovery mode at boot selection.
Then remove the drivers packages you installed typing at command panel:
yum remove akmod-catalyst xorg-x11-drv-catalyst
yum remove kmod-catalyst xorg-x11-drv-catalyst
yum remove xorg-x11-drv-catalyst-libs.i686
To make sure all are removed type also:
yum remove *catalyst*
I encoutered the same problem when I used the repository .rpm package drivers. ATI proprietor drivers will not give you nothing more than headaches at F16 with gnome. I installed it also using the ATI .run file downloaded from their site but gnome-shell keep crashing ramdomly (at least don't locked me off ^^), so I'm stucked with the open source ones, since the proprietor drivers from both repository .rpm and ATI Site .run can't seem to work well and have some weird issues.
Don't use proprietor drivers until ATI fix them to work properly with the last xorg and gnome shell (I believe gonna take long, they don't care much about us Linux customers).