This is quoted from the HSUS site, some interesting info about chipping:
"Microchips implanted in 2003 or earlier are generally readable by most shelters and veterinarians. Microchips that came into use in late 2003 are generally not readable by most shelters and veterinarians because the chips require different scanning technology. Microchip manufacturers have not yet provided shelters around the country with a scanner that reads all different types of microchips (called a "universal" scanner)
Each company that manufactures microchips has its own scanners, and some of these scanners can only "read" their own microchip. In other words, in some instances, the scanner of one company may not be able to detect the microchip of another manufacturer, which would indicate to the shelter staff that the lost animal is not microchipped. Without the ability to use one scanner for all types of microchips, shelter staff would have to scan the animal, who may be fearful and difficult to handle, multiple times with each manufacturer's scanner. Additionally, some companies provide their scanners free to shelters, some do not. Without sufficient numbers of free scanners available to equip all animal control vehicles as well as shelters, microchipped animals may go unscanned by agencies, which can't afford to purchase multiple scanners from multiple manufacturers."