[tlrobinson]

Theoretically the total cost of the energy expended on mining will be equal to the value of the mining rewards and fees received by the miners, minus the cost of the hardware amortized over time and a small profit.

Additionally, mining operations will gravitate to areas of the world with excess electricity, since it will be cheaper to mine and thus more profitable.

So the "cost" of running Bitcoin will rise with price, but fall as the block rewards continue to halve every 4 years (unless transaction fees make up the difference). The users essentially pay for it through inflation and transaction fees. It's really quite elegant.