According to the New York Post (NYP), Apple's VP of iTunes, Eddy Cue, has been in talks with major labels about a subscription model that would give customers unlimited access to music for a monthly fee. Details are few and far between, but NYP's sources suggest that the monthly cost could be around US$10-15. They also suggest that label representatives are receptive to the idea, as single-track sales have begun to decline.
Similar services like Rdio have gained a following by offering streaming and cached versions of favorite tracks for a small fee (Rdio starts at $4.99 per month). Spotify offers a similar service in the UK and is currently pushing for a US release.