As I understand it, we've got two main new usage sources of NG. One is electricity production. The other is fueling the commercial fleet. Fueling non-commercial vehicles-consumer cars and trucks still seems to be a stretch to me. The nice thing about the commercial fleet is that you can build out the refueling infrastructure with less expense. You can have a central refueling depot for big city fleets like busses and along the interstate refueling for trucks.
I think the ng movement will be about the 18 wheelers well before cars simply because the economics can be proven out very quickly.
Unless you are a long distance car driver and keep your cars for 65 or more years most of the economics do not pan out for any substitute to gasoline. Feel goodisms that the greenies try to use to get people to substitute on their cars are just that... feel goodisms. Until people are committed to long term keepers of their cars they just feel good but cost themselves thousands of dollars without any realization.