The main argument against labeling GMO foods comes from the companies themselves, who fear that these labels will scare consumers away, but it stems from a fear that if consumers know what companies are hiding in their food, they would move to other brands.
GMO crops are new enough that long-term health effects of their consumption cannot be fully known yet, that being said however, there are plenty of arguments on either side about possible effects they may have, and with the large possibilities of adverse health problems stemming from GMO crops, it seems absolutely necessary that our foods be labelled accordingly if they contain GMO's.
The US is one of few nations that doesn't label GMO products in food. Much of Europe and Asia either label GMO's, impose harsh restrictions on them, or outright ban them. Labels in the US would be an important first step in providing consumers with proper information and legislation is being proposed that would sign this into law. While the effects of GMO food is unknown as positive or negative, consumers have the right to know exactly what they are eating.