I am pretty sure that this has already been posted on ECOLOG, but since the topic has come up again...
A study by Patzek (2004) found that the use of ethanol (obtained from growing corn) ultimately consumes more fossil fuels than it saves. He advocates that the US dramatically reduce its corn production, and reduce its use of ethanol from corn. Not all biofuel comes from corn, but I suspect that the vast majority of it does. This is not the only study that came to this conclusion, but is among the most recent. It is a long read, but is in depth. Of course that does not make it right, but his argument is not trivial. Tad W Patzek. 2004. Thermodynamics of the Corn-Ethanol Biofuel Cycle. Critical Reviews in Plant Sciences. 23(6):519-567. Cheers, Shane Opinions expressed are those of the author and do not represent those of his employer.
