Turntable.fm is a lot of fun. I've been using it with offsite coworkers to create a sort of shared music space. Unfortunately, it seems like only a matter of time before it gets shut down by the RIAA or riddled with enough advertising to make it sickening.
It costs them around $.002 per listener of a serendipitous play, and 10 cents for the DJ (which is an on demand play)... This could get expensive pretty quickly, so I'd imagine they are negotiating/looking for another source.
Also interesting - to be DMCA compliant, they'll also have to follow other groundrules, so users:
1) May not see ahead in a playlist past the currently playing song
2) May only play or pause the list
3) Can only play 3 songs per artist per hour, maximum
4) Can only play 4 songs from a single album in three consecutive hours, maximum
5) May skip ahead only 6 times per hour.