A lot isn't available for OS X. You can get Unreal 3, but not UnrealEd. Or GECK. The main apps are quite often there, but the slightly obscure stuff that's more fun than the main app never is. You won't find a working level editor for Quake 3 either. I didn't, the GTKRadiant for OS X just doesn't work. Silent crash.
Generic use software like graphic design or media players are fairly abundant, and the situation is slowly improving, but anything for a single purpose in particular seems non-existant. Thank goodness for Boot Camp.
Athlon64 2.7gHz->OC 3.9gHz, 31C, MSi 9500GT->OC 1gHz core/2gHz memory, 48C, 4Gb DDR2 667, 500Gb Seagate + 80Gb Maxtor + 40Gb Maxtor = 620Gb, XP Home
Air cooled, total cost £160