IPods already let you load songs from multiple comps...you just have to make sure you turn of Automatic Sync, which is what wipes out your IPod completely if it finds that the songs on your IPod dont match the ones on the comp. you connect it to.
I dont know if this will catch on, myself, I could care less what extra features there are...all I want to be able to do is load music onto my mp3 player, im not looking to browse the web or buy shit direct off the mp3 player. If you wanna get on the net on the go, use a sidekick, or most phones already have that included..... so we will see. Microsoft is known to each up competition, like its starting to do with XBOX but this will take time.