H.264 acceleration on GPUs is now a reality, courtesy of ATI... if you have an X1x00 series GPU. This is just in time too for Blu-ray and/or HD-DVD, as these technologies have mandatory support the very compute-intensive H.264 codec.
To give you an example of how well this technology works, AnandTech has benchmarked a real world system decoding Apple's H.264 HD QuickTime videos. The H.264 speedup is impressive, and shows a significant decrease in CPU utilization during playback of these videos.
Unfortunately, the lower end X1300 GPU does not support H.264 acceleration above 576p. The X1600 or higher is required to decode 720p, which is disappointing since one would think that a 13.3" PowerBook with a 720p-compatible 1280x800 screen could get a Mobile X1300 within the year, but would not get an X1600 any time soon.