Though it's not the subscription service everyone's waiting for, Apple has announced the iTunes pass, in cooperation with EMI, which is an $18.99 service that will allow you to subscribe to automatic downloads of a certain band for a limited amount of time.
The first band to get the Pass treatment is Depeche Mode; if you sign up, you'll get two singles immediately, their new album when it is released in a couple of months, plus videos, remixes and other presumably exclusive content.
All material downloads automatically to your iTunes account, and you'll be notified of its presence each time, and thankfully, it's in the DRM-free iTunes Plus mode. In the case of Depeche Mode, it'll last 15 weeks, so I'd expect that to be around the same for other bands they decide to do this for, though I haven't yet heard what other artists might be "Plus-ed" soon.
I'm eager though, I'd consider signing up if one of my favorite bands were involved. Would you?