That sucks.
I think that's the only thing stopping me from getting a GPS. I live in an apartment, so I'd have to take it inside every night, and that's just not something I can see myself doing.
**edit- Um isn't there a way for the GPS companies to find your GPS, if it's registered? Or do anything? Seems like that would be easy. If not, call pawn shops. If they have it, they should have the ID of whoever sold it to them.