I was wondering about this also.
Not so much as complete replacement to the current OS, but perhaps ripping out the media player part of RockBox and using it instead of the current one.
well, rockbox does have its share of problems. and afaik the main things that makes solid-state mp3 players better for that task is the combination of specialized hardware, the lack of unnecessary features, and the inclusion of a custom lithium battery. i get over 10 hours of battery on my iriver...i'm sure it'd be a bit less if i had a color screen that was backlit 100% of the time, and i was always playing doom on it.