As a addition to this post, I must add that the Digital Angel device by Applied Digital Solutions was originally designed to be an implantable(!) device. They had stated this in their web site about the same time they recieved the patent for the device (about 1998 or so), but due to negative feedback from consumers, they stopped marketing it as such, and changed the site. They now make no mention of implants, yet, the research is likely continuing, esp. after Sept. 11.
Scary, huh?