Alex Deucher has been working on delivering full EXA Composite support for the Radeon driver and today the first bits of this work are available via his personal git repository. The full EXA Composite not only covers the R300 and R400 generations, but it already supports the Radeon X1000 (R500) graphics cards! In Alex's blog post
he mentions that some blend combinations still need to be debugged, but the R300/400 support in general is pretty solid. For the R500 support, full EXA compositing is working for some graphics cards but not others. For those interested in trying out this latest Radeon work, check out the r3xx-render branch
(well, until it gets merged to master). On the other side of the table, currently the RadeonHD driver lacks full EXA Composite support.