According to EA, Rock Band is no longer being considered just a game, they're calling it “a platform.” I would imagine that for the price we're going to end up paying, it's probably best to consider it to be an entire platform that will grow over the years.
During their press conference at E3 they stated that they will be releasing weekly updates after Rock Band is released. We're not talking just a few songs here and there, they are talking about releasing entire albums for download. The first one to be released is Who's Next by The Who. If you're strapped for cash, just only want a couple of songs, they can be purchased individually.
We've long heard people crying out to hear Metallica on one of the Guitar Hero games. While we don't know if we'll get our wish for Guitar Hero, Metallica will be heard on Rock Band. Enter Sandman will be included on the game when it ships, while other songs will be available to download later.
Ars at E3: EA promises weekly downloads for Rock Band, including entire albums [via opposablethumbs]