The guys at IntoMobile have posted this video demonstration of Rock Band for the iPhone, putting 4-way Bluetooth multiplayer to the test.
The multiplayer works very much the same way as it does in the console version, only without the bad singing. 4-players link their iPhones together using Bluetooth and go to town in any of the game's 20 included tracks. It's definitely less of a logistical mess than the console version, but I'm not sure I'm quite ready to see random groups of people on the streets jamming together on their iPhones. Not without the desperate urge to hit them.