DSLR Controller for Android/Canon is awesomehttp://market.android.com/details?id=eu.chainfire.dslrcontroller
Here is a (crappy) photo of a quick test at my desk. It could not be easier to setup (plug in the cable and you're done) and you have access to pretty much all the camera controls, including refined manual focus, histogram display and magnification.
I already have many ideas of when to use this application. With this I think I'm done squatting or bending for hours to take pictures. It looks like it would also be very useful at night or to do macro shot given how precisely you can set the focus.
The only drawback of the app is that the display refresh is slow. I don't know if that's a limitation of the 5D Mk II or the app itself.