There have been a few game controllers released for iOS devices like the iPad and iPhone, but none that I've seen that work as elegantly as the 60beat GamePad promises it will. The GamePad offers all of the buttons you'd expect on a HD generation video game controller (with analog sticks and triggers and so on), but instead of using the iOS device's 30-pin connector, it hooks up through the headphone port, and apparently passes audio information across there somehow.
Unfortunately, because that's such an interesting connection, the controller only works with two games at the moment,