Announced back in July 2009, Rock Band Network lets artists convert their music to tracks for the rhythm game and sell them through a storefront, receiving 30% of the sale in return. Track creation tools are free but a $99/year subscription to Microsoft's XNA Creators Club is required, as the system is built upon its framework.
Created tracks can be submitted for playtesting from the community and the "Peer Review" process--vetting for "vulgarity, adult content, legal concerns, etc."--that tracks pass through before hitting the Rock Band Network Store is similarly community-driven.
Once Rock Band Network launches, "songs will debut exclusively on Xbox 360 for 30 days" before "a selection of standout tracks" make the journey to Wii and PlayStation 3 too. The Rock Band Network Store is not currently in the beta but will be made available "in a few weeks," according to the official Harmonix Twitter account.
Harmonix and MTV Games have yet to announce when Rock Band Network will launch.