This is a wild idea and I don't know if its possible, but I'm putting it up here mostly so I remember to try this if I ever get some free time.
The idea is to write an iPad app that can connect to a Canon camera directly through the Apple camera connection kit USB interface. You'd have to reverse engineer the Canon USB control protocol, but these guys seem to have already done so and reimplemented the protocol for use on Arduino microcontrollers:
So I think this would be a matter of porting the USB control code to run on iPad, and then working on a GUI to control the camera with.
I'm not saying this will be easy, or if I'll ever have time to look into it. Add this to my pile of "DIY projects I would do if I had the time".