Using some conductive fabric and metal rods, you can drum on any modern touch device. There is an instructable to tell you how to make your own.
I built two apps for the sticks: iTaiko (for the iPhone) and iSteelPan (for the iPad). The steel pan app also has a whack-a-mole mode to teach you how to play.
iSteelPan with drumsticks from ayman on Vimeo.
In the iSteelPan app, the accelerometer is used as a proxy for pressure (softer strikes are somewhat attenuated).
Instructable: http://bit.ly/istick
Video demo: http://vimeo.com/11788466
Conductive Fabric
Metal
Cotton
Objective-C (iPad/iPhone Native)
Samples from: