There's been a lot of claims made about bioethanol on both sides of the fence, with some hailing it as the greatest thing since sliced bread and others claiming that it actually produces more GHGs than an equivalent amount of petreoleum.
This study at least appears to be a rigourous life-cycle analysis of the different types of ethanol plants (they identified 12 different types of plant). It's got quite an interesting history of the ethanol programme in the US (I didn't know that it started in 1980), and corn production (yields per acre are now eight times what they were 100 years ago).
Bottom line: Coal-fired ethanol plants do actually produce slightly more GHGs than gasoline, but on average corn ethanol reduces GHG emissions by 19% now, and is expected to improve to 21% by 2010.
Life-cycle energy and greenhouse gas emission impacts of different corn ethanol plant types, published by the Institute of Physics.