Scott, don't know where it stands now but Calif was considering a law that every diesel engine be retrofitted with an exhaust scrubber and hire a bunch of smog police to check.
What you don't hear about and that I feel will change everything emissions related is research currently in it's infancy breeding plants that pull greenhouse gases from the atmosphere, break them down to basic elements and return them to the soil. There is even hope that it will be able to reverse global warming.