Re-post:
The reason that the EU is getting a 451 diesel and we are not is the difference in required standards. Presently the EU requirement for NOx is =/< 2.6 grams per horsepower-hour. That reduces to =/< 1.2g/hp-hr for them by 2009. The 2007 US standard is =/< 1.2g/hp-hr, and reducing to =/< 0.2g/hp-hr by 2010. That is a significant difference between us. The additional standards for particulates between the EU and US are a lot closer, but the U.S. is still stricter.
As was alluded to elsewhere in this group, liquid urea injection is very effective in reducing NOx to our standard, but is only appropriate for larger vehicles. And the fact that the ratio of liquid urea (an expensive, HAZMAT item) to diesel fuel is 1:18 (cutting well into your miles per dollar). Service stations would have to install tanks, pumps, and nozzles, or sell cansiters to re-fill onboard urea tanks - not likely to be well met in the U.S. with so little interest in passenger diesels. There are emerging dry technologies, but they may also be too much bulk for a smart to carry.
Add: The new Canadian diesel standards will be very close to the U.S.'s.