→Hardware Video Decode Acceleration (EXPERIMENTAL)
If you have an older RadeonHD with UVD, you can use Catalyst 10-7 to get working acceleration, but users have confirmed acceleration broken since then.
red;>(EDIT Feb, 06 2013 : urls are dead. The first PPA doesn't have "catalysthack" anymore in repository, the second one is 404)</span>
For acceleration to work, you will need libva from this PPA: https://launchpad.net/~dtl131/+archive/catalysthacks and the xvba va-api backend from: http://www.splitted-desktop.com/~gbeauchesne/xvba-video/
With those installed, you can get acceleration from any video player that uses VA-API. A compatible version of VLC is available in the aforementioned PPA.