(MONEY Magazine) – Have you ever wondered where your money had been before it landed in your wallet? Boston computer consultant Hank Eskin did, so he started a website that tracks bills by their serial numbers: www.wheresgeorge.com.

That was in late 1998. Today more than 3.6 million bills are registered there--21 million dollars' worth in all. Where's George members enter a bill's serial number, their zip code and how they got hold of the tender. "My favorite stories," recalls Eskin, "are of bills that are spent at strip clubs and end up in collection plates."

Trouble is, some people--specifically, the feds--don't like seeing "www.wheresgeorge.com" on their money. Eskin sold a thousand rubber stamps of his site's URL to cover his costs. Purchasers have stamped bills with abandon. But defacing currency is a crime. So is advertising on it for profit, which brought the Secret Service calling on April 21.

"I knew they would come sooner or later, but I think the law is still debatable," said Eskin, who agreed to stop selling the stamps until a resolution can be reached. The Secret Service declined comment.

--BRIAN P. MURPHY