Oregon ran a pilot program in 2006 and 2007 that fitted 300 cars with GPS receivers, which kept track of the cars' mileage. The receivers also kept records of when the cars were on the road, noting whether they traveled during rush hour or not. When the drivers went to several specially-equipped gas stations, they paid a mileage tax based on how far they had driven and when they drove, rush hour being more expensive than the wee hours.
Taxing mileage -- as opposed to trying to raise fuel taxes -- is an idea that's not only raising eyebrows, it's also raising interest. Seven other states are reported to be interested in finding a publicly-palatable way to tax mileage. A panel in North Carolina even recommended that drivers be charged a quarter-cent-per-mile for their year's driving. In such a scenario, after 15,000 miles you'd owe the state $37.50.
Naturally, the hurdles are many and it will be years before we see anything like this happening -- but beware: it's gone beyond "Let's think about it" to "Let's look into this." States envision working with manufacturers to get the standardized mileage-reporting technology installed in cars. In addition to the substantial privacy issues that would raised by such a move, there's a question of whether a flat mileage tax would blunt the move to energy-efficient vehicles -- the gent in the Prius might not be happy about paying the same as the gent in the F-250 Super Duty.