It's called Soundbrush and for $1.99 you can combine your inner Picasso with your inner Mozart. Basically, you create music by swiping your finger (the "brush" in the app's name is apparently completely metaphorical).
A total of four instrumental lines are possible and you hear the notes as you draw. I can see this becoming rather addictive, although if you do buy the app, please spare the people around you in public places by using headphones.
If you have a kid, you already know how much the little tykes love your iPad, or as one of my friends calls it "a touch sensitive electronic babysitter device." This might be a way to get the kid into music sooner.