Katie has an international chip that was put in by her Canadian breeder. It has a fifteen digit number, and it showed up. My vet scanned her and gave me her number, and he told me it wasn't necessary to put in a US chip. It still scares me, though. I found this article written for vets interesting.
Microchip Fact Sheet :: Indoor Pet :: The Ohio State University
Katie's chip is endorsed by the Canadian Kennel Club, and it's a EIDAP chip.