Back of the envelope calculation for the U.S.: Using the national average gas tax of 48¢/gal, if you were to charge an electric vehicle an equivalent of what, say, a 30 mpg vehicle would pay per mile, it'd be 1.5¢/mile, or $150/yr for a vehicle that does 10,000 miles a year. I don't think that's really a huge change to the overall economics either way, though it could push some marginal cases from above to below breakeven.

I visited New Zealand recently and was told that the reason Diesel was so much cheaper than unleaded was because it didn't have the road tax built into it so agricultural vehicles wouldn't have to pay for road they weren't using. As a result Diesel cars had to keep a logbook and pay a tax based on their mileage. Didn't ever get to see it in action.

Flying cars