I bet its getting interesting... but i doubt propane is the final solution: It increases the combustion temperature, which lowers soot emission but increases the need for either EGR or DEF....
sadly, people are herd animals - particularly here. if enough there is enough media coverage to say that propane, natural gas (or whatever flavor of the the month is) will be cheaper than what they are paying now, then they will buy it hook line and sinker. this can, and probably will be, accomplished by whoever can profit from it. in the end we'll pay more and get less out of it. ten years later we'll realize we were hoodwinked. it doesn't have anything to do with what is better or cheaper - just how much money can be made from it.