Thanks to all who replied. My sense of this is, as said above, that it is early in the game at least in the US. I gather that having a "certified" conversion done is expensive, and may be limited by law to some new vehicles--at least older classic vehicles appear to be few and far between as conversions, perhaps relegated to eye candy for their advertising and "gee-whiz" value.
I am not sure that I want to be the first on my block to run the zepplin on an alternative fuel, but there are enough long haul trucks being built to run on NG (in its various forms) to think that there might be a chance that much more will be done in the future. Boone Pickens, the oil investor, appears to be involved in CLNE (Clean Energy Fuels), a company which is putting fueling stations at truck stops on some interstates. At the current price of the fuel ($2-$2.50), they claim that the payback for 18 wheelers is measured in months. But at current rates of build, it will be more than a decade before NG fueling stations become anything like common beside gas and diesel pumps, and that is by no means guaranteed to happen. It is probably unlikely that any significant local, state, or federal funding will be made available to move this along at a faster rate.
That is what I think I have learned--
Best to all,