Virtual buttons
From Openmoko
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 using a button. That way, it is possible to create virtual buttons on the device without making holes in it.
This can be useful for simple actions, like increasing/decreasing the volume, or so.