The battle over whether or not law enforcement agencies can legally use a GPS tracking device to monitor your activities is still raging in the US court system. Right now animal rights and environmental activists are being surveilled with these devices; it's only a matter of time before cypherpunks who have come in from the cold, lawyers, and privacy and anonymity advocates come under the watchful eye of Big Brother for exercising their First Amendment Rights. To complicate matters, lower courts scattered around the United States all have different opinions on the practice, so a few are hoping that the cases before the Supreme Court will straighten things out (ideally, in the favor of Joe and Jane Walking Down the Street). An interesting bit is that, when challenged in court over ownership the FBI basically writes it off and stops trying to get it back.
This means that a couple of enterprising hackers felt safe tearing one of them apart to analyze how that particular model works. Long story short, they are modular devices built out of parts that you can probably buy off the shelf if you've a mind to. The FBI apparently had theirs custom built by a contractor. You can buy those battery packs on the surplus market for a decent price, GPS receivers with serial interfaces can be bought for a couple of bucks each (I think mine cost about $10us, plus shipping), and you can buy single chip radio transmitters for a few bucks from a good electronics supplier or eBay (just keep in mind that you may need an amateur radio license to legally use them). Or you could buy a pre-paid cellphone and hack it to send text messages every few minutes with its current GPS coordinates.
It's somehow reassuring to know exactly what one might have to deal with one day. If it bleeds, so sayeth a certain movie, you can kill it. Perhaps in the twenty-first century, that saying should be updated to "If it runs on electricity, you can jam it."