A few points regarding the acceleration:
It's not precisely hardware, and AFAIK there's nothing to help with DivX/MPEG4 (someone correct me if I'm wrong!), but it can offload the MP3 decoding onto the DSP, which gets loaded with its own tasks. This can also be done for Vorbis and FLAC, at least, and conceivably others.

One interaction to note, though; when the DSP is doing anything, the CPU drops to 333 MHz; this is because the DSP runs slower at max CPU frequency (266 MHz is out of DSP spec):
So when dealing with heavy video, better performance is possible by dropping the DSP back to 133 (still enough to decode mp3 realtime), and boosting the CPU to 400. Unfortunately, this cannot be done automatically, at present, and isn't horribly easy by hand.