And then of course, the security vs civil liberty problems. The government will be able to track to you to tax you, but the police will not be able to track you? Yeah right. Do you think criminals are stupid? Either they will tamper with the GPS, drive old beaters that don't have one, or they'll mug people on foot. It's just another way to keep ordinary folks in line.
Remember -- only 10% of anything can be in the top 10%.