For those using EXA acceleration with this open-source ATI driver, this feature can be enabled through the EXAVSync option in your xorg.conf. At the same time, Bi-cubic X-Video filtering for the R300/400 series has also been fixed.
Right now if you want this support you'll need to check out the latest Git master code for xf86-video-ati until a new release is made available. Additional details are available from Alex's blog. Further commentary can be found on David Airlie's blog.