True story - one time when I was working a temp gig, one of the full time employees heard I was good with computers and asked me if there was a way I could hook her up with a GPS tracker for her car so she could find out where her husband is when she's at work.
I did a bit of research [http://www.divorcenet.com/states/new_jersey/spying_on_your_spouse] on that and found out that there's actually quite a few online Internet firms that offer to sell such a device, however, they usually caution in their disclaimers that if you get caught using that device and face criminal charges for invading their privacy that they are not to be held accountable.
So, these guys offering to implant a tracker in people's teeth? Basically offering to help you perform a borderline illegal activity. A lot of them might even be a scam - if they take your money and bail, who are you going to tell? The cops? You'd basically be turning yourself in.
And the answer to the question is always, "Don't worry, I'm just keeping your girlfriend warmed up for you."