for the most part the vegi oil is dumped however they can get rid of it. It does harden like you said, but there are 2 ways to avoid that and they recomend using both methods together.
One is heat the fuel with an in-line heater or a heated tank.
My car left the factory with an in-line heater so I'm set there.
And they also recomend using a little bit of gas in the fuel. They have charts for the amount, but it isn't much
Here's a couple of links on the subject
http://journeytoforever.org/biodiesel_svo.html
http://www.dieselveg.com/
http://www.lowimpact.org/course%20ou...conversion.htm
There's plenty more with a google search