Not quite the whole truth Nora.
Film of 400 speed or slower requires roughly 4 passes through a carry-on x-ray scanner to become fogged. Film of 800 speed or faster can be damaged by a single pass through the carry-on x-ray scanner. These estimates vary slightly depending on the make and formulation of the film, how old it is, how it has been stored, and most importanly, the make and model of the x-ray scanner (not all put out exactly the same amount of x-ray energy).
Film of ANY speed WILL BE COMPLETELY RUINED by even ONE pass through the checked bag scanners. The checked bag scanners are much more powerful than those used on carry-ons and WILL RUIN ANY SPEED FILM with just one pass.
Please read through my page on the subject before you pack up your film for travel:
http://www.willcad.org/image_film_tips.html