Confirmed in an interview at IGN
The important bits:
The important bits:
IGN: The most important question. Will Rock Band 1 DLC be playable in Rock Band 2?
Dan Teasdale: Yes. We've been developing Rock Band as a music platform from the start. All of the songs that you've purchased so far (and will be able to purchase in the future) are instantly playable and fully supported in Rock Band 2
IGN: Once RB2 is out, can you still download and play new tracks on RB1?
Teasdale: Absolutely! We've been working hard with Microsoft and Sony to develop a way to share content between games, and we're really happy that we'll be the first game to support fully functional cross-title DLC.
As far as Harmonix is concerned, there's no difference between "Rock Band 1 DLC" and "Rock Band 2 DLC" – we're just releasing awesome songs every week onto the Rock Band platform, all of which are playable by the Rock Band titles that support DLC.
IGN: Are any improvements being made to the instruments? And will we get wireless guitars for 360?
Teasdale: [...] And yes, all of your Rock Band instruments will still work with Rock Band 2.
We're also actively welcoming new peripheral makers to make instruments for our game. Just like real instruments, we want players to be able to choose what instruments they use, and we'll be expanding the choice of players alongside Rock Band 2.