Whenever questions like this come up, I try a quick search in google.com and usualy come up with good results.

An article last year by George Will reported than as of 1975, 80% of Americans had never flown.
http://www.townhall.com/columnists/g...20010802.shtml

A Washington Post/ABC poll conducted on 9/13/01 showed that 10% have never flown on commercial airliners. Another 35% flew less than once per year. Oddly, 1% of respondents had "no opinion" as to whether they'd flown before.

http://www.washingtonpost.com/wp-srv...data091401.htm

Margin for error is +/- 4%.