As the price of petrol fuel supplies slowly moves upward due to demand from China and India, I foresee datacenters moving away from diesel generators as backup power sources towards fuel cells/generators that can burn natural gas and hydrogen.
Technically fuel cells don't burn the fuel; they rely on a chemical process that is rather like a battery except that it relies on a continuous supply of fresh fuel to supply hydrogen atoms. The fuel cell has a catalyst which strips electrons off the fuel, and those flowing electrons are electricity. Unfortunately, most fuel cells run very hot (600 C with molten carbonate) or use noxious chemicals (sulphuric acid slurry). But a lot of work is being done into developing fuel cells that run at low temperatures and which are not so fussy about the fuel that you feed them. One company that has interesting commercial products right now is Acumentrics: http://www.acumentrics.com/products-power-generators.htm --Michael Dillon