Google Music support was just announced for the Android Market. You can preview and buy music right on your phone! Right now you are probably asking "How can I get it?!"
Well, normally we would have an APK download for you, but this update is totally server side. Anyone running Market 3.3.11 is ready to use the Music Market. Google is it out to accounts right now. If it doesn't work for you, just have to wait. Sorry.
Mine did update though, So I might as well show you some screenshots.
It looks exactly like the market does now, the music section is just orange.