I heard about this on the latest episode of "The Skeptics' Guide to the
Universe" podcast (http://www.theskepticsguide.org).

"Exposure to low levels of radon appears to reduce the risk of lung cancer,
new study finds"

"Exposure to levels of radon gas typically found in 90 percent of American
homes appears to reduce the risk of developing lung cancer by as much as 60
percent, according to a study published in the March 2008 issue of the
journal Health Physics. The finding differs significantly from the results
of previous case-control studies of the effects of low-level radon exposure,
which have detected a slightly elevated lung cancer risk (but without
statistical significance) or no risk at all."

http://www.physorg.com/news125672761.html