The scanners used to screen carry-on luggage are not likely to fog your film. Notice I said "are not likely", not "can't". Federal law (I can quote you the chapter and verse) states that you have the right to have the TSA hand check your film. Some people have reported signs at check-points that seem to imply that they will only hand check higher speed films that are more sensitive to X-rays, but they are misleading. Most signs also state that film of speeds less than 800 ISO is safe, however photographic industry testing has detected problems with fogging of film at speeds as low as 400 ISO. So, the bottom line is that the safest thing to do is to ask for a hand check. To speed things along, open your film boxes and canisters and dump the rolls into a Ziplock bag for easy inspection.
Also, never place film in checked luggage. If the airport uses Invision CTX explosives detection scanners, it will fog your film... of any speed.