Reported by USATODAY, Honda is aiming to release a diesel vehicle that meets US Department of Energy Tier 2 Bin 5 emissions standards:

As for the diesel, Honda Vice President Ed Cohen said the timing is right to bring a new breed of quieter, smoother, cleaner engines to the USA. They achieve 30% better mileage than comparable gas engines. Volkswagen and DaimlerChrysler also have made a commitment to diesel in the USA.

Their technology is called the 2.2L i-CTDi and is already said to be more efficient, with more torque and less noise and emissions than gas and even the hybrid engines. Watch a nice Honda introductory video here, and read a comparison of honda engines (from Dec 2004!) here.