It's a keyboard. With a nob on the side of it.
Scheduled to see Rock Band 3 at E3 next week? Now you don't have to! USA today's published a large video of the game, showing off the new keyboard, drop-in gameplay, confirmation of full backwards compatibility and more.
There's a large preview of it here, detailing what's "new and notable".
Just watch it. Looks cool. It's out this year.