A few months ago, my spouse had her wallet stolen. Amazingly, after she and two bystanders chased the thief over several city blocks, he was caught. By that time, he had thrown the wallet away, keeping only the cash. But, when the police asked if she could identify her cash, she said, "Actually, I noticed last night that one bill had a WheresGeorge.com stamp on it."
Thanks to some WheresGeorge.com member out there, we may some day be able to recover this bill. Right now it is in "evidence" with the District Attorney. If we do manage to get it back, we will certainly enter it into the site, along with a note about this incident.
I never thought those markings were a good idea, but I changed my mind after this happened. I've started entering all my bills because I now realize I can use the WheresGeorge.com site to identify currency in case of theft.