Si estan usando la versión 12.4 del Catalyst, tienen que revertir a una versión anterior, pues 12.4 al parecer no es compatible con el Diablo III.

I've been notified that the current Catalyst 12.4 release drivers aren't going to be supported for use in Diablo III. Either revert to an earlier release or maybe the 12.5 release

As of right now, tonight, the word I have is 12.4 will not be supported on release. That could change between now and then though.

Catalyst 12.4 drivers aren't supported. You'll need to revert to an earlier version. 12.2 can be found here: http://support.amd.com/us/gpudownload/windows/Pages/radeonaiw_vista64.aspx

I did a little checking after I last posted and apparently, the 12.4 release is having enough issues right now that we're going to post that warning you saw if they're detected in use. The best solution, again for now, is to revert to a slightly older release. This is subject to change at any time, but for now, it looks like it's going to continue on into release.

Fuentes
http://us.battle.net/d3/en/forum/topic/4837103904
http://us.battle.net/d3/en/forum/topic/4837543907
http://support.amd.com/us/gpudownload/windows/Pages/radeonaiw_vista64.aspx