Biodiesel is in limited use in many cities, most typically in buses. Cooking oils, once filtered and dewatered, are particularly useful, even though you may think you are following a rolling BurgerKing. Some is derived from wood and other vegetable waste, but mostly on pilot plant scale. Other posters are correct....it is a matter of current economics. As to diesel engine cleanliness, you can look forward to big improvements by 2006, when North American refineries must all have completed process changes to reduce sulphur in diesel from current 500 ppm to (something under 10 ppm I think......I'm too lazy to look it up again). Europe has been at this low level for years. It is the sulphur content that prevents engine design that gets to potential clean engine emissions, and in particular has prevented application of catalytic converters on diesel. Note that it is a pre-requirement that all supplies be upgraded before engine conversions are practical, because one load of dirty fuel would plug or contaminate the modified engines. We'll see dramatic decreases in carbon particulate through the same engine technologies. I think either Edmonton or Calgary (Alberta) has a special test run this winter on its fleet of buses, converted to use the clean fuel and fitted with catalytic converters just like cars. News reports say the results are dramatic.