Over the last several years, builders have been required to rough in radon mitigation systems in new homes. This way, should high levels of radon be discovered, it's not too difficult to complete the installation of the radon vent. That pipe would be connected to a fan and then vented outdoors, and the fan would pull the radon gas from under your basement floor to the home's exterior to keep you safe.
How can you be sure your shouldn't be a functional radon mitigation system? http://www.moneypit.com/audio-q-a/how-do-radon-testhttp://www.moneypit.com/audio-q-a/how-do-radon-testhttp://www.moneypit.com/audio-q-a/how-do-radon-test. I recommend picking up one or two charcoal adsorpotion canisters, and follow the directions for testing. Generally these are left in the lowest living area of the home for period of 2-7 days, then sent to a third party lab for analysis. If radon levels exceed 4 picoCuries per liter of air, then completing that radon mitigation system would be warranted.