I was at Google I/O, and I am very curious to see how this works. I like the idea of my music in the cloud, but streaming to my phone will cut its short battery life even shorter. I sure hope they have some kind of syncing or caching component as well. Also, they'll need a mobile client that could easily mix both…

Grooveshark has been around for a few years now, though maybe they have some new services now - I'll have to check them out again.