Interesting concept, Huck.  Would it apply to electricity sales as well?

Here's the problem I see with it.  As people adopt lower BTU lives, such as buying a more efficient vehicle, increasing the house efficiency, etc. the amount of revenue goes down.  And as it goes down, the government would need to increase the BTU mill rate to maintain revenue.  But the folks who are too poor to buy that hybrid car or move into downtown get disproportionately hammered by increasing taxes while the well-off can afford to upgrade their homes and buy energy efficient stuff.

Your solution needs to include some component for helping people invest in efficiency.