According to this IGN report, Rock Band 2 will ship for 360 this September, with "other platforms" hitting later in the year.
While all peripherals from the first game will work with the sequel, upgraded versions will be available in a bundle pack. Harmonix isn't revealing what these upgrades are just yet.
"As successful as the original Rock Band continues to be, we've now had the benefit of the last eight months to listen to our fans' requests and to build upon that foundation," said Alex Rigopulos, co-founder and CEO of Harmonix. "Rock Band 2 is a second-generation band game that will elevate the music game experience to the next level."
There's a Q&A with lead designer Dan Teasdale here.