The Air Guitar Move is an accessory for the iPhone that's basically a guitar pick-shaped sensor that connects via the dock connector. The guitar pick part has a motion sensor and sends data to the iPhone, where various apps combine the sensor input with on-screen game controls to form the rest of the virtual guitar. You use the pick to strike the imaginary strings, just as you would with an air guitar.
It certainly beats touchscreen-based Guitar Hero games for authenticity, and it's a clever idea that I'm sure will interest many wannabe-guitarists. You can currently preorder the Air Guitar Move for $39 through Kickstarter, and you'll only be charged if the project reaches its $25,000 funding goal within the next 24 days. Alternatively, you can pick up the Elago guitar pick-shaped capacitive stylus for $9.99 and jam out in any of the virtual guitar apps that iOS has plenty of.