Harmonix just confirmed that Rock Band 3 will release this holiday season.
Said the firm on Rock Band's Facebook page:
"Harmonix is developing Rock Band 3 for worldwide release this holiday season! The game, which will be published by MTV Games and distributed by Electronic Arts, will innovate and revolutionize the music genre once again, just as Harmonix did with the original Rock Band, Rock Band 2 and The Beatles: Rock Band. Stay tuned for more details!"
There's no more detail right now, but keep watching for updates from GDC.