We are working on a deal with some local animal processing facilities to take their waste, use it to make methane, compress it and use CNG to fuel their fleets. It works best for vehicles that drive set routes and are coming by the same place all the time to fill up so you don't have to worry about finding places to fill up.
On first blush, the economics are attractive. Gasoline is currently running about $30 per MMBtu and diesel is about $28 per MMBtu. Our CNG would cost about $ 4 per MMBtu (higher than NG these days but also take care of waste streams they are paying to get rid of).
Unfortunately, it is very expensive to convert an existing vehicle to run on CNG and even more expensive to buy a new one--about $80k to convert, so the payback isn't instantaneous. It is still pretty attractive for the processors, though.