Here's a game for the iPhone and iPod Touch that should appeal to the more rhythmically-inclined iDevice users out there. It's called BeatRider, and while on the surface it looks just like another Guitar Hero clone, it actually has an ace up its sleeve.
BeatRider's main selling point is that it allows users to play using their own MP3s. To do that, players will have to upload the song they want to play to the BeatRider servers since apparently existing App Store apps can't directly access a user's music library. That's a feature that's supposedly going to be introduced in the iPhone 3.0 firmware, though, so it may not have to be such a roundabout process in the future.
Here's a video showing the upload process as well as some actual gameplay at the 2:50 mark. BeatRider is currently awaiting approval for an App Store stint. There will be a free Lite version as well as a paid download.