From ArsTechnica Ford's plug-in hybrids will talk to electrical grid This is for charging the cars at the best time of day (night), but in theory could be extended to a means for charging cars for electricity different than regular electricity, in other words, a mechanism for replacing the gas tax with a different energy input tax.
Fuel (or electricity) taxes are input taxes. Theory suggests it would be better to tax outputs (actual miles traveled, by time of day and location). This would send a more direct signal to consumers about the costs they impose on the system and others. The difficulty is that this may be a much more difficult enterprise from a variety of points-of-view (collection costs, political acceptability, and even technology (GPS shadows etc.). As a second-best, input taxes are not too bad, it is better than a tax totally unrelated to usage, and the 20-30% reduction in collection costs may well make up for any inefficiencies.