I think you are moving a little ahead of the times with this! True it is possible at this time but not very cheap and stable. The only idea that COULD work is the wireless internet (Ricochet). But that wouldn't solve any of your problems and create new ones.
1. You would still need hard drives for your "Desktop collection"
2. 128K really wouldn't be the best bandwidth to be working at (less than CD quality). Plus think of Ricochet as a cell phone... Even in the biggest cities there are holes in the coverage, and the signal strength varies. Well the same will apply for Ricochet. Weak signal = choppy music. No signal = no music.
3. "capacity to hold practically unlimited mp3 songs" Nothing like this out in the market today, and I doubt in the next decade.
In all it is a great idea, and sorry to shoot you down but I this it is too early for this to happen economicly right now. You could just use the wireless connection to connect to your desktop and the internet, but as far as streaming MP3 the bandwidth just doesn't cut it right now. But I could be wrong.