The Rock Band iPhone app has been released, and is priced at $10. That may seem a bit steep for a iPhone app, but since the game includes twenty songs, that's actually a decent deal for the music alone.
Four player co-op is supported via Bluetooth. The game is controlled by tapping the screen, which I'm sure feels more like drumming than strummng. Oddly enough, even singing is controlled the same way. That could be good or bad, depending on the quality of your co-op partner's voice. Song packs are also available, coming at the price of two songs for a buck.
Funk #49 by The James Gang / Let Love Rule by Lenny Kravitz I Was Wrong by Social Distortion / Girl U Want by DEVO The Kill by 30 Seconds to Mars / Here It Goes Again by OK Go This Is A Call by the Foo Fighters / It Hurts by Angels & Airwaves Today / Zero by the Smashing Pumpkins