California's Proposition 37, if it passes, will require labels on GM foods. Michael Specter defends the safety of such foods:

The safety of genetically engineered foods has been studied hundreds of times over the past two decades. The National Academy of Sciences, the British Royal Academy, and scores of other scientific bodies have concluded they are no more dangerous than other foods. There has never been a single case of a person becoming demonstrably sick as a result of eating genetically engineered food. The American Medical Association has said that G.E. crops “pose no new or different risks than any other crop, and there is no scientific reason to believe they would be any more risky.”

The board of the American Association for the Advancement of Science released a letter in which it stated that “the science is quite clear: crop improvement by the modern molecular techniques of biotechnology is safe.” The letter also quotes from a recent report issued by the European Union, where the use of genetically engineered foods causes violent opposition. “The main conclusion to be drawn from the efforts of more than 130 research projects, covering a period of more than 25 years of research and involving more than 500 independent research groups, is that biotechnology, and in particular GMOs, are not per se more risky than e.g. conventional plant breeding technologies.”

Plumer adds:

So what’s the best argument against GM labeling laws? What can they hurt? Critics of Proposition 37 have argued that the law is poorly crafted and would unfairly burden small businesses and retailers. Here’s Kevin Drum: “This initiative, as with so many initiatives, is sloppily written; it can’t be changed after it’s passed; and it imposes expensive state labeling burdens on interstate commerce.” Drum also points out that California went through a similar experience with toxic-chemical labeling laws. “In the end, so many warning signs got posted that they became essentially useless.” 

Meanwhile, U.C. Berkeley’s David Zilberman worries that labeling laws will “create a stigma effect” that will hinder future research into using GM foods to improve nutrition or tackle climate change. Remember, genetic modification is a tool that can in theory be used for a variety of purposes — not just bolstering Monsanto’s bottom line. See this essay by Pamela Ronald for how plant genetics could play a key role in sustainable agriculture. Note, though, that this is currently a very tiny slice of actual, existing research into GM foods.