Said the company through its the official Rock Band Facebook:
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!
The news apparently emerged while Viacom--owner of Harmonix parent company MTV Games--was discussing its upcoming products, going by The Wall Street Journal.
No other details were revealed, though there have been indications that Rock Band 3 could feature music by Jimi Hendrix and possibly make use of the Xbox 360's motion-sensitive Project Natal add-on. As for platforms, PlayStation 3, Xbox 360 and Wii editions are likely, though a PlayStation 2 version is unclear as the two most recent RB games--The Beatles and Green Day--have not and will not hit the aging platform.