Here's the link.
The gist: It's cheaper than Napster or Rhapsody, especially annually, but it doesn't work with the iPod and there's no way to test-drive the service and software without signing up for an account, which I don't like. It's free for the first seven days, but still. I want to see how the software works, what the stream speeds are like, what bitrate and file format the downloadable tracks are, how good the radio stations are, how easily I can search, etc, what artists they have that the other services don't (iTunes doesn't have the Beatles, Zeppelin, or Radiohead, for example, or the Stone's 60s catalog). So I guess we'll have to find some guinea pigs. I'll poke around to see if there are any reviews yet.
Also note that the service is in beta stage. They don't specify what that means, though.
Edit: Ah, it turns out that you can download the "Yahoo Music Engine" without signing up for an account. Shows what I know.