Be careful when talking biodiesel. You often need to define what you mean by the term. Technically, Biodiesel is vegetable oil that has been through a refining process which adds methanol and lye to remove all the glycerin and other stuff. Vegetable oil (often called VO, StraightVO - virgin oil, or WasteVO - from fryers) congeals at a very high temperature. If you're using WVO and it has an animal fat in it, that will solidify at room temp. Obviously this does terrible things to your fuel pump, fuel filter, and injectors. If you want to run VO (often called a greasecar) you need a separate fuel system to hold the VO. The tank, fuel lines, fuel filter, etc need to be heated (this is often done by running engine coolant around these systems). Of course the coolant isn't warm until the engine is warm so you have to start the motor w/ D2, let it warm up, then switch over. Also before shutting the engine off you need to switch back to D2 so the VO won't clog up the injectors, filter, pump, etc. as the engine cools down. But on the plus side you can just take WVO from restaurants, filter out all the solid bits, and you have fuel. VERY cheap.
Biodiesel on the other hand is much more stable although you still have to be careful if you want to run B100 (100% biodiesel) in colder climates. Depening on the feed stock (soy, canola, etc) the cloud/gel point is different and standard D2 anti-gel additives don't work. So most people don't run B100 below about 40F.
Biodiesel and D2 can be mixed at will and B20 is a popular level where you get some benefits of bio w/o worrying about gelling and such. Personally I've started "splash blending" my CRD. I had a 55-gallon drum of biodiesel delivered last week and I filled the tank w/ 3/4 D2 and topped off w/ B100 so I'm running roughly B25. I'll run this way for a couple tanks then move to B50 and ultimately to B100 by next spring. The place I'm buying from here in the Bay Area charges $3.25/gallon w/ all road taxes paid and delivered in a drum. Considering D2 is $3.39/galon here rt now it's a easy choice. I also like the convenience of having fuel at home in case of an earthquake or whatever.
So biodiesel is a great thing - it might even be cheaper than D2 in your area and it's a hell of a lot better for the environment, not to mention we can grow it here at home and don't need OPEC. Being able to run biodiesel and towing performance were the main resons I bought my CRD. Sorry, my Northern California tree-hugger is coming out
For more information, check out the government's biodiesel website:
http://www.biodiesel.org/ and
http://www.biodieselnow.com
Be ready for some serious hippy-speak, Mother Earth and all that. But it's folks like Willie who are making biodiesel acceptable for regular folks.
Try it out!