XBOX360 UK got the lead designer in an interview, and he focused mainly on cross-platform concerns. Will you be able to play downloaded Rock Band songs on both Rock Band 1 and 2? Yes, you will. All Rock Band Songs will be playable on all Rock Bands.
He also offers non-commital superlatives about the new Rock Band having the best soundtrack ever. But, the more interesting bit to me is what you can deduce from the following two quotes:
I'm pretty sure John Drake (Harmonix PR Coordinator) will hit me if I confirm or deny anything, but I can say this: We've taken almost everything people have asked for in Rock Band and added it to Rock Band 2, as well as including some "first time ever" features.
Harmonix believes that user creativity and user-generated content has an important role to play in music games especially since it is a company made of musicians – either in currently touring bands or in the past notable ones -- and we strongly believe in the possibility of giving every-day people the chance to create their own music.
Possible composing mode!? Look out lame, lame, lame Beatles. Your songs are no longer safe from the real ravages of fake rock and roll.
UPDATE: notriously curmudgeounly AC/DC will be in Rock Band 2!