The Street Orchestra iPhone app is like a simple version of Rock Band that lets you play classical music with up to 200,000 synched iPhones. Simple, classy, and free.
Basically what you're doing is gathering up a bunch of folks with iPhones, having them download the app, pick instruments, follow on-screen instructions to get everyone in-synch, and then starting the game.
The volume of individual iPhones will adjust with the player's in-game accuracy. Better game skills mean a louder instrument.
The app's free and available in the iTunes store now. [iTunes—Thanks, Jerry!]