And dollar is backed by nothing but trust and actually dollar is nothing but numbers in computers: some of dollars has paper form but it is minority as far as I know. I am not saying that you are not right. I personally see why bitcoin might succeed. By some aspects it already succeeded having in mind that their total value is 200 milion dollars (am I right?). Question is if people will trust more centralized currencies or decentralized.

Energy backed currencies? Energy is in a constant state of flux from low to high-entropy states. And 50-100 years from now, energy will be the one thing that isn't in short supply, given that the sun sends more of it our way per year than we've used in human history: see http://www.sandia.gov/~jytsao/Solar%20FAQs.pdf - page 10: 480 exajoules (EJ) strikes the Earth per 90 minutes. Convert that to the yearly number, and compare to worldwide energy consumption in the year 2001 of 430 EJ, bearing in mind that we didn't use much before the 19th century.

So you're a SMAC player? http://en.wikipedia.org/wiki/Sid_Meiers_Alpha_Centauri

I'm sure Firaxis wasn't the first to think of this idea - but it does make sense as energy has intrinsic utility and value. If not pure energy, then definitely currency backed by resources that create energy (ie, solar farms, gas/coal mines, nuke plants, etc).