The reason that compressed natural gas isn’t a good idea for truckers lies in the trucker’s contract with the company they work for, not in the actual price of CNG.  This article over at Seeking Alpha explains it pretty well.  What it boils down to is this: trucking contracts are written in such a way that they protect the trucker from variations in gas prices, so a cheaper fuel isn’t going to help the trucker, and in some cases actually hurts.

It’s too bad, because if truckers would convert over to CNG there would be a huge demand for it.