already installed one
I installed a GPS tracker/Telematics unit to my car as an anti-theft measure way back in 2004... (a modified Maxxon TrakM8) This has been coming for a long while, and isn't all negative...
After an accident, I used it to prove that the other driver was at fault, was lying about what happened, had pulled out without looking (directly into my path), and that I wasn't speeding...
Before that, it'd knocked around 10% off my insurance premiums as the car could be tracked via GPS, and I'd built an add-on to the system that could remotely disable the engine should it be stolen, and trigger a camera built into the dash so that a record of who had stolen the car was available...
This was all before 3G was widely available, so it was all done on just plain old GSM. With the 3G and WiFi that's available now, ideas such as these could make car theft much less common, and accident disputes almost impossible meaning that insurance costs should come down...
Limiting the speed of a car via GPS is a very dangerous idea though... I've had to break the speed limit in the past on numerous occasions to avoid accidents, or get away from idiot drivers. Having the car decide that I was breaking the law (based on a calculated GPS location, which isn't always 100% as we all know!) without knowing all of the various other things going on (that electronic sensors can't sense!) would cause more problems that it'd help...