Old, but decent basic article describing how emissions regs. work in the US.
Note that regs. are fuel neutral. Light vehicle diesels and gas fueled engines comply with same standards but also note standards are a fleet-average standard, not individual vehicle standard.
http://www.edmunds.com/car-technolog...gulations.html
Info on TDI emissions controls, worth reading if you want to ponder and speculate on the potential complexity of any fix or ways emissions compliance cert. might have been gamed.
http://www.natef.org/NATEF/media/NAT...-0-TDI-SSP.pdf