Some of you have asked about diesel hybrids. From what I can see, any sluggishness in bringing the such a platform to market in the US has more to do with the markets perception of diesel than any technical limitations.
Seattle and New York City are both bringing in diesel hybrid transit buses. From the October, 2003 Seattle announcement:
King County...which includes Seattle, has announced plans to purchase 235 diesel electric hybrid buses, the largest ever order for this kind of urban bus technology.
The 60-foot articulated vehicles use the E System Parallel hybrid powertrain developed by General Motorss Allison Transmission, and are powered by Caterpillar C9 ACERT engines. They will be supplied in 2004 and 2005 by New Flyer Industries, which is based in Winnipeg, Manitoba.
The hybrids are expected to be 20-30% more fuel-efficient than a standard bus, and operators are likely to recover the cost of buying them over their 12-year service life. Diesel particulate filters will reduce PM emission by about 90%, while the vehicles smaller engines are expected to reduce NOx by about 50%.
Here is a good overview of NYCs program from the DOE.
GM delivered a prototype diesel hybrid Silverado tactical vehicle to the Army last year. (Bringing a whole new meaning to lean mean green fighting machine.)
Mack Trucks is developing hybrid diesels for military and commercial vehicles.
DaimlerChrysler is offering a diesel Mercedes E320 in the US this year; the company has shown a production diesel hybrid version (2005 model year) of its Vision Grand Sports Tourer.
And there is some interesting research and results on diesel hybrid technology that Ill pull together.