We'll have more details as they are announced.
[Update: It will "not be a Rock Band title; it will be a custom game," according to Rigopulos. It sounds like it will be visually different, and not just a Beatles skin for Rock Band. It is planned for next year's holiday season. In addition, Harmonix is "not ready to talk about" Rock Band interoperability.]
[Update 2: they're "not prepared to discuss" new peripherals.]
[Update 3: they're also not prepared to specify the platforms on which the new game will be released.]
[Update 4: specifics of the music included in the game are not being discussed, but Harmonix wants to include music from the "span" of the Beatles' career.]