A top ranking post today reports the following factoid:
The amount of corn it takes to produce 75 litres of ethanol– roughly a tank of fuel– is enough corn to feed one person on a 2,000-calorie-a-day diet for a year, Mr. Klein said.
But clearly we’re not all going to be just eating corn for a year. Some people actually eat meat too… so part of that corn needs to go to cows:
For every 3,000 calories in the form of corn that are fed to a cow, only 600 are returned in milk; if the meat is eaten, only 120 calories are available for human use. – Source
Put those two together, and you end up with a choice between a tank of bio-gas or 15 days of steak. Chicken is twice as efficient (per Michael Pollan) so you could have a month of poultry for the same corn cost.