According to the Rock Band Facebook page,:

Rock Band 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 backwards compatibility is a must for this game, and I expect it will be available. One new feature I’m sure they will include, is the ability to play multiple instruments of the same type, as they just allowed with Band Hero. Multiple people playing the drums, or singing the same song would be a great addition to the game. Any one else have some interesting ideas for the upcoming Rockband 3?