http://www.techreport.com/onearticle.x/12552
Direct words from AMD: "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."
So basically it's similar to Nvidia's 8800 series of cards (only partial hardware video acceleration). Which makes a perfect sense - why waste extra transistors on this feature (full hardware video acceleration) if majority of 2900XT users are likely to have a CPU powerful enough to do most of the decoding in software? And HTPC users are highly unlikely to buy such expensive, hot, power-hungry, noisy card anyway.