Rockband [$9.99 - iTunes link] for iPhone and iPod touch is now available in the iTunes App Store. It has guitar, bass, drums, and vocals, and both single-player and multi-player (via Bluetooth) action.
20 songs come with the app, including music by Smashing Pumpkins, Lynyrd Skynyrd, Foo Fighters, Blink-182, The Beastie Boys, and more can be purchased in-app.
If you get your Rockband on, let us know how it works for you!
[Thanks everyone who sent this in!]