Unless you have been living in a cave for the last week, you know that the new version of the iPhone is out. The iPhone 3G. The other thing that is out is new software for previous iPhone/iPod Touch owners. The new version of the system software supports applications. There are lots of games and stuff out. But there are TWO music ones that are worth a look.
The first one is Karajan Beginner. Theory trainer and stuff. There is a paid version of this as well which I imagine expands on the drills.
The second one is GuitarToolkit. Yes, it is more geared towards guitar players, but the tuner and the metronome are excellent. I thought it was a good $10 spend for a tuner and metronome, though it would be nice if you flipped the iPhone over, it would flip the tuner over. Perhaps the next version.