Harmonix and MTV Games have announced some new upcoming DLC for Rock Band 3, which includes John Lennon's classic album Imagine. The companies are also planning to add tracks by Bon Jovi and Billy Joel soon after the game has been released. Exactly which songs from those two artists that will be available hasn't been made official yet, all that is said is that it will be "packs of some of the greatest hits".
Our review of Rock Band 3 is available right here. Spoiler: we like it. A lot.