While some granite counters have been found to give off trace amounts of radon, very few granite countertops pose a real threat. The EPA has even issued a statement saying that there is insufficient evidence to suggest that granite countertops are a source of radon. EPA lists a safety limit of 4 picocuries per liter of air so if you are worried, pick up a home testing kit for about $25. If you see levels above the EPA's limit, call in a specialist.
I have included a few web links that explain this topic in more detail.