True, slacker can auto find music for you like Last.FM but it's so much more. Go to the website, and give it at least 15 bands. Once you do that, name and save your station you can access it on the iPhone. From there, you only hear songs from those bands. Slacker provides a lot more control. Plus, it's rumered to have streaming audio quality that Pandora or Last.FM can't touch. Other things you can control:
-You can ban or heart songs
-You can control how often hearted songs are heard. Play them a little more, or tons
-You can play new, old or in between music
-You can play the popular songs, the hits, or the fringe
-You can control how often you hear "artist discovery" songs, all the way from never to lots
I think the app is going to be free. It's not going to save music to the iPhone, just stream, but I hope it's a really good start.
Also similar to Last FM (
http://www.last.fm) ... basically music recommendations based on your current "likes"