Enter the iControlPad, a device that wraps itself around your iPhone and provides some tactile controls to software (we posted about it back in May, but we're closer to seeing an actual release than ever). Touch Arcade's got more on how it works, and pictures of another rumored device (though no manufacturer is mentioned). Button presses are delivered via the serial port, and jailbroken apps already support the pad, with full code and SDK support to come.
Pretty interesting -- iControlPad wants to sell theirs for less than $30, and of course if the idea takes off, they'll have some competition (maybe even from Apple?). The iPhone is already a great gaming device -- would a button-based peripheral make it even better?