The current mobile version of Rock Band features 25 licensed songs from artists like the Beastie Boys and Smashing Pumpkins. Plus, there's multiplayer jam sessions and the ability to perform lead bass, drums, guitar & vocals.
No telling how much — if any — of that will make it onto the iPhone version. Guess we'll just have to wait and see what EA Mobile says beyond their ambiguous Twitter announcement.
Note: This screen is from the Rock Band Mobile game.