According to the Bureau of Transportation Statistics, there were 617,977,711 passengers on domestic flights in 2009 [1].

At the (incredibly optimistic) risk rate of 1 in 30 million, that's about 20 people given cancer, each year, if this policy goes forward and these devices become standard procedure.

[1] http://www.transtats.bts.gov/Data_Elements.aspx?Data=1