So I've been playing with a few of these services (see last post) and so far they all seem to fall short.
1. Avvenu: Great interface, but needs an upload monitoring progress bar (which is still slow) and track by track upload functionality
2. Mediamaster: Great upload speed, great interface, cool widget but playback is a little esoteric; limited opportunities to purchase.
3. Tagworld: Doesn't quite work, not sure why; this is more of a social networking site anyway
4. Sideload: interesting desktop sync function but still kind of slow
5. Tunefeed.com/Faces.com: Decent widget but painfully slow upload; a little "too" robust with tons of feature creep.
We're in the early stages of virtual locker for music use only because it takes too long, the interfaces are still clunky, and virtual vs. physical storage distinction isn't that great. But it could change...
What would be interesting? Partnering with last.fm (which does have radio widgets) or slacker to provide recommended tracks for purchase and sharing within one's virtual locker. But that seems obvious. More interesting: adding a tagging function ala delicious or digg for tracks found on the web, which would then be recorded in the virtual locker (again for purchase, no piracy bullshit here). This preference data would then be posted to see which ones are being shared the most.
More interesting, and obviously more complicated, would be identifying top 3-5 song groupings. Because some tracks, like Snow Patrol's Chasing Cars and Death Cab's Someday You Will be Loved, would tend to be together (hypothetically).