That depends on the sensitivity of the film and the strength of the X-ray imager.
100 ISO through carry on once? Probably no issue. Instead of once, 8x? You'll see things. ISO 1600 through carry on once - you'll see lines. 100 on checked luggage? It's gone.
100 ISO through carry on once? Probably no issue. Instead of once, 8x? You'll see things. ISO 1600 through carry on once - you'll see lines. 100 on checked luggage? It's gone.
edit: Kodak posted some examples with X-ray scanning awhile back: http://support.en.kodak.com/app/answers/detail/a_id/30482/~/... - it really depends on the sensitivity - and that effect is additive.