If by some random chance you typed in the serial number of a GPS unit you bought used on eBay or in a pawn shop into Google and this page comes up, you have my GPS unit (Serial #107813400 -Garmin Nuvi 350). I don't want it back - you seriously keep it, but I do really want to know where you bought it from.
I want to catch whomever smashed my wife's car window making me pay for not only a new GPS and mount, but also a new car window. And since there have been several GPS thefts from my apartment complex as of late, I want to prevent this guy from doing it again.
Update: September 2010
Probably due to the good search rankings this post gets, for example searching for [stolen gps], a number of people have added comments with their own experiences and a few more serial numbers. I'll add them here for better search visibility, although I don't intend to keep this updated often: 1C0462207, 1Q163077, 76902542, 1Q6051977.
One user, KarenR, mentioned in the comments that Garmin will let you register the serial number with them and if they ever get it in for service or a map update, they will not perform the service and will send the customer information "over to someone for further action.", presumably law enforcement. Your mileage may vary.