Off the top of my head, I would think you accomplish this with out an arduino.
Sketched out, I would simply have a PIC, a switching device (Maybe a Normally Open relay) and an old keyboard.
The 'switching device' being an object that when the PIC goes LOW. will create a contact closure.
Rip apart the old keyboard until you just have the circuit board (usually containing the Num, Caps, and Scroll Lock LED's) Figure out what combination of pins will result in a 'Left Shift Key' or 'Right Shift Key'.
Then with some wiring, When the motion sensor is tripped, it will go through the switching device. and trigger the 'shift' key. Only glitch to all this is every time the motion sensor is tripped, the shift key will be pushed. But you can always wire it to a random other key like F12.