How is this Coke promo going to work? If it's really GPS and not RFID, the cans are going to need to have a GPS receiver and some sort of transmitter in them. I'm assuming the reporter is misinformed- as RFID is the trendy evil tracking device that requires very little to work. If it is GPS- the GPS satellite system is a one way deal. GPS satellites transmit- they don't receive locations. The cans would need to send their location info another way- cell or paging networks maybe? They'd also have to have their prize patrol vans very close- so someone at Coke would have some idea where the tracker cans where... and as we know from the McDonald's Monopoly scams- when employees know where the "prize piece" is located, someone is going to rip-off the system.
Either way- tracking who buys Cokes is just a little scary, but if it's the beginning of marketers watching what and where you buy- then compiling the info into a database of your purchasing habits- to build more effective marketing campaigns targeted against you... Then that's very scary.