Ethanol produces more carbon dioxide than petrol.
Both petrol and ethanol produce carbon dioxide when burnt. But to produce the same energy requires more ethanol than petrol, and it produces more carbon dioxide.
Moreover producing ethanol from corn by fermentation also produces more carbon dioxide during the fermentation process. And then the dilute ethanol produced by fermentation (about 12%) has to be concentrated using a distillation process which requires more energy and more emissions. Burning ethanol rather than petrol thus probably increases the production of carbon dioxide.
For detailed calculations by Professor Jim Barrante see: http://carbon-sense.com/wp-content/uploads/2010/11/petroleum-v-ethanol.pdf [PDF, 38 KB]
James R. Barrante is an Emeritus Professor in the Chemistry Department at Southern Connecticut State University. He has devoted his last ten years to the study of the physical chemistry of greenhouse gases. He is the author of two textbooks, Physical Chemistry for the Life Sciences (in Japanese) and Applied Mathematics for Physical Chemistry.