Virtual buttons

I have seen a digital camera with this feature: the accelerameters can be used to detect small shocks on the case. If one taps his fingers on one corner of the device, a program can find where the tap was done and can interpret that like if there was a button in that corner. That way, it is possible to create virtual buttons on the device without making holes in its case.

This can be useful for simple actions, like increasing/decreasing the volume, or so.