Turns out the Californian SULEV standard is very strict. I found the following presentation concerning Advanced Diesel Engines for the EU and US auto markets by Prof Dr.-Ing. Peter Hofbauer.
If this interests you, check page 12 (for current standards), 24 (for an idea what can be done) and the last one (for sheer fun).
Essentially, Diesel in North America cannot achieve even ULEV standards unless Sulphur content goes below 10 ppm, as the NOx Traps would burn out at anything higher.