I posted the registry entry needed to enable it ages ago. Search and you should find it. There is a chance that it changed in the latest ATI drivers with the new interface. I don't rotate mine, so I didn't care.. .just when I found it (looking for other options) thought others might want it.
btw, its default enabled in XP and disabled in 98. But works in both.
This is only to show the rotation tab itself.