The new version of Slacker for iPhone's been buffed and polished so it glistens with a better UI, "4x the song library as our leading competitor" (Pandora) and faster transitions. Look for it tomorrow. [Slacker]
If Slacker has 4x the song library of Pandora (and I get five songs per month) then I would be interested.
But it would have to have something similar to Pandora's ability to intelligently personalize. The hugest song library doesn't matter to me if I have to spend the entire time (that I'm listening) looking for the next song.