I still think that leaving a valuable phone lying around (something which could easily be stolen and sold for money) is not very smart in a situation where you know there is a thief.
The best way to avoid being the victim of a theif, in my opinion, is to make yourself a more difficult target. If you know money is being stolen, I presume you know where the money is being taken from (since you want to put a camera there). You could just not leave money there. However, continuing to leave money there, and then purposefully leaving your phone nearby, seems like a very foolish solution.
Edit: If you insist on going the "surveillance" route, a simple search for motion detector on the Android market will lead you to several apps that claim to do what you want.
