This great news. As you guys already said, I'm waiting to hear about the drums (they probably just don't know yet), and if those work then I'll happily buy the disc version of RB2 and probably a bit of DLC as well. But since I'm mainly a guitar player and singer, there's a chance I might buy RB2's disc even without drum support.
Instrument compatibility is long overdue and is a great example of the sort of situation where the hardware manufacturers (Nintendo, Sony, and Microsoft) can and should step in to enforce certain interface regulations to make sure their customers don't get screwed.