Ok maybe it's not really much of a surprise. In any case, IGN reports that Rock Band 2 is in fact in development, and will be released this September for Xbox 360. Other platforms will follow later in the year.

Improvements in Rock Band 2 include a tracklist comprised entirely of master recordings, easier party-play setup, better quality instruments, and an expanded World Tour campaign mode among other things.

DLC released for Rock Band 1 will be compatible with Rock Band 2, and DLC released after Rock Band 2 comes out will continue to work with the original game. Downloadable songs do work differently in Rock Band 2 however, with lead designer Dan Teasdale teasing that Harmonix has "added some cool new features that let you play your DLC in different ways".

So why would you want Rock Band 2? According to Teasdale, "Rock Band 2 takes all the lessons we've learned from making a multi-instrument music game, addresses them, then adds some amazing new ways for you to experience your music library that have never been seen in a music game before. We've been scouring forums and parties everywhere for what people liked and didn't like, and I'm pretty confident we've addressed everything I've heard people ask for."