Actually, that idea wouldn't be very hard at all... at least in Europe. You can run an internet connection over power lines. So, when you plug in the car, it connects to the internet, authenticates with the dmv, and then logs how much power you're using and how far you drove.
A lot of cars are now coming with bluetooth connections. Why not put in some sort of 802.11 connection? Come home and plug your car in, it logs your mileage, maximum speed, average speed, mileage efficiency, and syncs your MP3 library.
Granted, you can make the argument that not everyone has internet or wireless access points, but I'm willing to bet that the overwhelming majority of people with electric cars do. You could even include a modem that dials in over your phone line or bluetooth to your cell...
You can still say there are the exceptionally poor that don't have a phone, cell, internet, etc... but then they're probably driving a gas car or using public transit.