I get an email every couple of weeks from somebody that has found Where's Tim and wants me to help find their lost cell phone. I always reply nicely and tell them that it is not possible to track a random phone via GPS based on it's number or ESN. First of all, if it was that easy to track something, I would be out of a job. Second of all, it goes against everything we believe in as far as allowing the trackee (is that a word) to be in full control of who watches them and when. Thirdly, it shows a mis-education of how GPS works. People have seen too many movies where the bad guy throws a magnetic tracking device on the back of the good guy's car and then track them down later. Those devices exist in concept, but they are bulky, expensive and you would not casually toss it at a passing car and expect the driver not to notice. Not to mention that even if you could get past the ethical dilemma of tracking somebody with out their permission, it's illegal and considered an invasion of privacy.
I'll talk more later about profitable, positive uses of real time GPS.