low power accelerometers are very expensive. You could rig-up a piezo shock detector, though, for the sort of feature you're chasing.
You're right about price. I don't really mind, though, as I don't plan to mass produce or anything.
I think the piezo would require a lot more design considerations. It would also be larger.
With the accelerometer, all the fine tuning can be done in software.
I also had the idea to use it to log sleep patterns. Graphs could be shown right on the display in the morning.
edit: I may be wrong about the piezo. Do you think the flexible film type could be adhered to the back of the display and function as described. It wont flex much behind the layers of glass.