Gundato: Bit questionable (the 'jacking of the music), but sounds nifty.
How does it stream music? Does it stream actual radio stations (if so, which stations :p) or is it like Pandora/Last.FM where it generates a station for you?
Actually, it's more like Grooveshark. You choose whatever artist and song you want to listen to, and it streams you the song. At the same time, you can hit a button on the website or mobile device and have it sent there. The songs can be played only through the app, but the app responds just like the regular Music app does on iPhone, so you can do other things while listening to music. You don't have to stay connected to listen to the song as long as you click the "Sync to mobile option" and connect to wi-fi or through your cellular network once and a while to confirm you're still subscribed. In a way, it's kind of like Zune Pass except you can use the service over a broader range of electronic devices. The website also makes it really easy to put songs on the device. You don't even need to connect the device to a computer!
If you're still curious, they have a free 3 day trial that doesn't require you to enter credit card information or anything.