This Scientific American article explores the purported benefits of corn-based plastics (also known as polylactic acid, or PLA) over the more conventional petroleum-based plastic. The results may surprise you.
Although PLA is technically renewable, saves oil, and is less polluting to manufacture, its biodegradability is far from perfect. It's also a headache for recyclers because PLA "contaminates" the conventional plastic recycling process. The two must be separated, leading to higher costs for recycled plastic and lower overall demand.
Some experts have also suggested that the marketing of PLA as "green" plastic has led to rampant overpackaging and a proliferation of single-serving plastic containers.
A related 2006 Smithsonian Magazine article can be found here.