An anonymous tipster seems to have contacted several Mac rumor publications with rumors about an unlimited music subscription within iTunes.
According to the e-mail, Apple will charge U.S.-based customers $130 per year (or $100 for MobileMe subscribers) for an “iTunes Unlimited” subscription starting in late October that will include the ability to download about half of the songs in the iTunes store in a 256-Kbps format -- presumably AAC files that are protected by an updated, subscription-capable version of Apple's Fairplay DRM. The rumored subscription would only include half of the songs in the store because offering music under a subscription plan requires a new deal with copyright holders.
Subscription songs would be playable within iTunes as well as on certain supported devices -- most likely the iPhone, iPod touch and other upcoming iPod models. However, older iPods would most likely be incapable of playing the files, because they almost certainly lack necessary technology to verify the validity of a subscription.