Colorless, odorless and undetectable by your average human, it is none the less the second leading cause of lung cancer in the United States.
How widespread is the problem? Radon has been found in homes in all 50 states. Certain areas are more susceptible than others (http://www.epa.gov/radon/zonemap.html), but no location is immune.
The EPA web site (http://www.epa.gov/radon/manufact.html) provides information on finding an appropriate resources and testing devices.
For more information, visit the EPA web site on radon at http://www.epa.gov/radon.