Radeon 2600HD, late 2007 iMac with 1920x1200 24" display.

With 3.2 and 3.3 kernels (haven't tried other versions) Catalyst 12.2 and 12.3 peg the CPU while running glxgears.

This is with vsync enabled and glxgears returning FPS of 60.

Catalyst 12.1 does not exhibit this issue.