Micro-chipping does not guarantee that your pet will not run away, or even that he/she will be returned if that does happen. It means that there is a sliver of hope that if your pet runs away, they may return, perhaps even ten years later.
Fredericksburg resident Gary Rowley found himself in this situation a few weeks ago after receiving a Facebook message from a mom in Oklahoma, whose children found his dog, Brindle in a bush by their home. After taking him to the vet, they discovered he was had a microchip ID, directing them toward Virginia where his owner still lives.
There is some debate over the safety of micro chipping because of a series of veterinary and toxicology studies, dating to the mid-1990s, which stated that chip implants had "induced" malignant tumors in some lab mice and rats. However, the American Veterinary Medical Association said in a 2007 statement:
“Considering how a large number of pets have been implanted with microchips with a relatively small number of confirmed cases of tumors associated with microchips, the AVMA advises against a rush to judgment on the technology.”

The Washington Humane Society suggests micro-chipping pets because of the increased rate of pet-owner reunions and a decrease in euthanasia because of reunification.

According to a recent study in the Journal of American Veterinary Medicine, shelter officials were able to find the owners of lost micro-chipped pets in nearly three out of four cases.

We figure that the benefits out way the potential risks of micro-chipping. Let us know what you think!
To read the story about Gary Rowley:
http://www.kswo.com/Global/story.asp?S=11283385