Might be cool. I'm sure there'd have to be some DRM stuff or something. Maybe something like how you authorize your computer to play iTunes songs/videos, same thing can be done w/ iPods & iPhones. Wouldn't like it, but would help w/ the legal issues. The two devices can see which accounts the other(s) is authorized for. If they're authorized for the same account, they can share the DRM'ed music. If not, too bad. Also, maybe multiplayer games would be cool.
Also, I'd like to see people be able to use & download stuff from the iTunes Store on their iPods/iPhones. In another thread, someone said that if you leave the wi-fi spot before you're finished, what happens? Maybe the download would stop and you can continue when you're reconnected to the internet. iTunes already has this or a similar feature in iTunes. Just go to to Store -> Check for purchases. Anything you haven't downloaded yet can be downloaded. Hopefully, Apple will add a feature where you can redownload stuff just in case you lose it. Also, be able to take songs & videos from your iPod to your computer through iTunes. That would really come in handy.
A built-in microphone would be good, too. Students can record lectures and listen back over & over. Would be cool if you can take video, too since a lot of lecturers write stuff on the board or do something that audio won't pick up. But that might make things too complicated or would totally suck.