See:
http://www.epa.gov/radon/
http://en.wikipedia.org/wiki/Radon
How much credibility you give such assessments is up to each individual. A lot of the concern will probably have to do with what part of the country the house is located. Also, if there are enclosed areas (such as a basement) where there isn't a lot of air exchange, that can increase the likelihood of radon gas accumulating.
The county where we recently built in OH is well known for radon issues. We had the home inspected after construction was completed and as expected, it tested above the standard. As part of the construction and the contract with the builder, the house was built with the appropriate piping installed from the basement up to the attic along with a separate electrical circuit and receptacle in the attic so that installing the mitigation system would be very straightforward. Works well and now tests well under the specified standard.
Bill