A GMO labeling law has arrived in the US, albeit one that has no teeth (link). For those who don't want to click on the link, the law is passed in haste to pre-empt a more stringent Vermont law. The federal law defines GMO narrowly, businesses do not need to put word labels on packages (they can, for example, provide an 800-number), and violaters will not be punished.
One of the arguments against GMO labeling is that it is unscientific because (some) scientists are 100% certain that GMO foods are safe. (e.g. this Boston Globe editorial)
Any good scientist knows that scientific "truths" are true until they are proven otherwise. Science is a continuous process of making hypotheses, and finding data to confirm or reject them. The Bayesian way of thinking is very useful here. Being true is a probability - more confirmatory data increases the probability that a given hypothesis is true.
So why is GMO labeling good science?
In fact, I'd go so far as to say that there is no science without GMO labeling.
How is nutritional science done today? What is the research that tells us coffee is good, butter is good, salt is bad, etc.? Granted, this is a shaky field that has issued lots of false results. But the usual form of analysis goes like this: conduct a large survey of consumers and ask them about their diet (e.g. how much red meat do you eat each week?); obtain information about their health status, either through the same survey, a different survey, or direct measurements if they are part of a research study; then correlate the dietary data and the health data.
Now, imagine you want to study if eating GMO foods affects your health, either positively or negatively. Your survey question will be something along the lines of "How much GMO foods did you eat last week?"
Without GMO labeling, there is no way to conduct such research. This is why GMO labeling is good science. Not labeling GMO is bad science - actually it mandates no science.