Here's a map revealing states that ban local counties from enacting seed legislation. This lobbying move occurred after Mendocino and Marin counties in California successfully legislated bans on GMO seeds in 2004. Agbribusiness got to work to prevent further such actions around the country. Here's a map laying out the activity (in red), from Enviromental Commons.
Environmental commons also details the status of these laws.
Not surprisingly, the laws have been passed in key agriculture states where the planting of GMO corn and soybeans are prevalent. An attempt to pass such a bill failed in California in 2006.