Radeon HD 2900 XT lacks UVD video acceleration
True to my word, I set out yesterday to test HD video decode acceleration on a Radeon HD 2900 XT using an HD DVD drive and a version of PowerDVD supplied by AMD for such purposes. To my surprise, CPU utilization during playback on our Core 2 Extreme X6800 test system ran between 40 and 50%, well above what one would expect from a solution with full HD video decode acceleration.

Naturally, I contacted AMD to inquire about the problem. I received a reply from AMD's David Baumann discussing the issue that ended with this revelation:

Be aware, though, that HD 2900 XT itself does not feature UVD, this is specific to 2600 and 2400, so the levels of CPU utilization you see should be somewhat similar to the previous generation.
http://techreport.com/onearticle.x/12552