It looks like those Xbox LIVE individual music stores that have been teased since the last update are finally hitting today.
Microsoft has announced that stores for Rock Band, Lips, and Guitar Hero should be available for download today. The first store, Guitar Hero VIP Pass, is a standalone program that points to all available downloadable content for the Hero titles. In functionality, it looks and feels a heck of a lot like the Rock Band music store which already exists in-game.
Personally, I find these to be welcome additions. As someone who purchases a lot of Rock Band DLC (yes, I'm guilty of spending way more money than I should), not having to pop the disc in first to browse, preview, and download the content