The attendant would probably have an override switch that would cause the pump to revert to a per gallon tax.
In fact, that might be how they would get people to go along with this: raise the gas tax exhorbitantly, like say $1.50 per gallon, with the milage based system as a "voluntary" alternative. That way they could get out of the state paying for retrofitting older vehicles and the privacy complaints, saying that the consumer voluntarily bought the electronics to save on the tax.
I forgot to mention that in that scenario, the attendant wouldn't even have to override the system. If the pump detects and can converse with the GPS, it charges based on the mileage. If not, the huge per gallon tax goes into effect (stick it to the out-of-staters, who have no choice, dodge a couple political pitfalls, and discourage people from disabling their GPS's). Nifty neato, huh?