there's a few problems i can see with a gps tracker. first, it has to be hidden somewhere on a bike. and it has to be serviceable, like changing a battery, so the only two places possible would be in the handlebars or the seat post. it wouldn't work inside a wheel because it would blow the tire, or be damaged from the forces applied to a wheel. any other external hiding place would be easily discovered, and you'd be able to track down a trash can where the thing was dumped.
in most cases, it'd only work in the few hours after the bike was initially stolen, because a thief would most likely strip the bike to sell it, and would probably discover it, or he would dump the bike, since a lot of thefts are of convenience, where someone needed a ride, and took yours.
another concern would be the transmission of the actual signal. if the bike was kept in a garage, or indoors, the reciever wouldn't pick it up.
still another problem, is bike recovery isn't high on any law enforcement's to do list. it'd be up to you to track your own bike down, and hopefully not get beat up in a confrontation, or get caught hiding in the bushes outside of someone's house.
the best thing to do is get a good lock, lock it somewhere secure and in public, and since you're in california, register it with the serial number of the bike as the vin, and engrave a number on the engine and use that.
supposedly, the bike goes into a data base, so if anyone gets busted riding it, the cops can run it and find out.
when i got a ticket for not wearing a DOT helmet, the cop wrote down the serial number of my bike on the ticket. i didn't have the registration for it yet, so who knows if he woulda actually ran the numbers, but at least you could report it stolen...
check it out: 
