Mongo pointed me here, so I thought I'd respond to a few things.
Only old engines need any modifications to handle biodiesel (it will eat away at natural rubber, which isn't used anymore).
I run b20 to b40 all winter (20% bio/80% diesel or 30% bio and 70% diesel) in New England and have never once had my fuel gel up.
Bio is WAY better than ethanol. Ethanol has a 1.29 to 1 energy ratio, meaning for every 1.29 gallons, it takes 1 gallon to make (barely worth it). Biodiesel, when grown from soy or canola, is a 3.2 to 1 energy ratio. And, of course, the ratio goes way up when you're using waste veggie oil that would have been composed or dumped in the past.
But by far, the best bio is still not in wide production - biodiesel from ALGAE! We can get incredible (hundreds of times better than veggie oil) yields of it, using only water and sun (and salt water will do).
Biodiesel is about as clean a fuel as you can run. It's carbon neutral (no global warming, thanks), and has way less soot and other emissions. In fact, the only emission that it's not good at is NOx. BUT, we can take care of that by tuning the engine a certain way.... but we can't currently tune the engines that way because there's too much sulfur in regular diesel and diesel vehicles would NEVER EVER pass inspection if they were tuned to lower NOx.
So, when we get clean petroleum diesel (2006), then we can get way cleaner diesels in general, and a diesel running on bio will be the cleanest car on the road. The only way a car could be cleaner is if it was electric, running on solar power.
Plus, biodiesel requires no supporting of oppressive regimes that support terrorism and keep their own people down. It stops the hemorraging of money from the domestic economy (1/3 of our trade deficit is petroleum), and it reduces our dependence on foreign countries!