Ah, yes, you need XGL, ATI does not yet support AIGLX (or the other way around). XGL works great in Gutsy, it installs easily (apt-get xserver-xgl). The problem is that OpenGL doesn't seem to work under Xgl so you either get 3D apps or a 3D desktop for now. Wait a little longer. I'd like to get off Xgl as well since it's not very stable yet.