Being on the tube/commuting for ~ 1 1/2 hours a day or so and seeing >6 ipods through glancing for white buds alone, the possiblities are huge.
What are net connections used mostly for (in terms of Mb up/down) It's P2P. There wouldn't be any roaming charges, any peak rates. You could do it in a lecture room, whilst you were studying, or having coffee with friends (sharing tunes, rather than listening )
Think one big interacting social darknet

Think virality without PC's needed.
Someone has a cool tune, and it could replicate exponentially!
For more benefits: Linking up to USB wireless receiver chips - you can wireless move files to/from PC.
Hands free driving - using changeable function paddles/butons on the steeering wheel. Hell - You could have a HUD of iTunes on a car soon (or at the very least, hook it up to those screens in the back of those orrible 4x4s )
In terms of illegal possibilities, think discogs. The amount of music you'll bump into increases a lot, so the rarer stuff might be out there. You could strike up a friendship with someone who had say, the entire back catalogue of (insert your fave band/movie/TV series). People could be walking lossless discographies of current artists. A discog of an artist is at most probably under 10Gig, so for a >60Gig player...
Who needs radio when you can stream? You could get it to actively hunt for a MP3 id tag genre - rock/pop, or highly rated artists. You could have the function to hunt for certain artists/songs...
That's another reason why I want wireless.