Diesels do not need to be any more polluting than gasoline vehicles.
Before the big push for exhaust cleanup in the late 1970s, diesel emissions were naturally much cleaner than gasoline emissions. The only reason gasoline is cleaner now is because a huge amount of R&D money has been spent on cleaning up gasoline exhausts, to the point of having to resort to using platinum, palladium and ruthenium based catalysts to clean it up - some of the rarest and most expensive metals on earth!
However, diesel engines are only now beginning to receive the same attention, and great progress is being made.
Ricardo here in the UK have just announced a SULEV diesel engine, which makes it much cleaner than existing Tier 2 bin 5 limits.
http://www.greencarcongress.com/2007...o-announc.html