Well, yes and no. Ethanol, without by-products, from corn is barely feasible. The value of the gluten or DDG's (as well as the existence of cash and futures markets in corn) make corn ethanol quite feasible.
And much better renewable fuels then Ethanol.
If a farmer is looking to maximize his subsidy income, growing corn for Ethanol is a good idea for him. If he is looking to maximize his BTU production, growing rapeseed (or another oil crop) for BioDiesel is a much better idea.
Farmers pay the bills with dollars, not BTUs, so they grow corn for Ethanol.