via iLounge
The Zune has it, but no one owns a Zune. Rhapsody, Napster, and a few others carry subscription services, though the combination of different devices and software often conspire to make the service a hassle. The CEO of Intent Media Works (a household name) has had meetings with Apple and claims the iTunes provider is considering a service offering all you can eat music for a monthly fee (presumably with DRM and an expiration date).
Some claim that subscription services just aren't popular with the teens who buy singles and aren't even sure what an album is, but with Apple's clout perhaps the long player can make a comeback. Then again its sales seem to be just fine so it's doubtful this will come to pass unless one of its competitors picks up some momentum.