Anita Ramasastry has written more than a few interesting privacy-related columns for FindLaw. Her most recent one, Tracking Every Move You Make Can Car Rental Companies Use Technology to Monitor Our Driving? discusses the use of GPS as a monitoring technology, particularly in rental cars. There have been a few cases in the US and Ramasastry dicusses how GPS can be used, with clear notice and consent.
Thanks to beSpacific for the link: beSpacific: Rental Cars, GPS Tracking and Your Privacy.