Woohoo! I might have gotten something right!

Have you thought about using something like this:ADXL345 3-axis Digital Tilt Sensor Acceleration Module. I know it's for Arduino but I'm sure a similar solution could be forund for PBP and they're fairly cheap but I was too lazy to look for one. They probably aren't as cheap as your vibration sensor though.

I did find some two axis tilt sensors but if you're doing an alarm you might want the three axis and I think an accel would work well. The math for it looks daunting http://www.freescale.com/files/senso...ote/AN3461.pdf. You could also use IOC with the accels which I think would be easier by just looking for a raw voltage change. I realize this system probably won't be used in the National Art Gallery but it could be made to be fairly sensitive using accelerometers.

Good luck.