I happened to read this short news story on Popular Mechanic's website:

"As if there weren't enough mixed messages from the auto industry and false promises about a hybrid future (click here for complete coverage), it turns out drivers can get a run-in with the law for running their cars on biofuel. North Carolina resident Bob Teixeira found that out the hard way after he converted his 1981 diesel Mercedes to run on vegetable oil and ended up facing hefty fines. Although Teixeira paid more for his fuel of choice in order to avoid dependence on imported oil (click here for our number crunch on alt-fuels), what he wasn’t paying was the highway fuel tax of 30 cents per gallon, which nets the state $1.2 billion annually to pay for highway expenses. Now Teixera owes $1000 to the state, expects a $1000 fine from the federal government and has been told he will have to pay a $2500 bond if he wants to continue using grease for gas. —Nick Mokey"

What a crock! How did they even find out he wasn't buying fuel from the pump? Were they spying on him?! Anyway, since when can a state force you to pay a bond for finding and making your own fuel, for your own consumption? Ridiculous...