Old news: there are some new X-ray machines at many airports (esp. the CTX-5000) which are very good at spotting dangerous items but also pose a high risk to any films that might happen to be in your checked baggage.
These machines are usually used on checked baggage, so it is much better to actually take all your films with you in your hand-luggage. Of course, a conventional hand-check is even better: the X-ray effect is additive, which means that if you are using the plane several times on a trip, even relatively low speed films may suffer.
The damage may be quite dramatic, but it also could just cause a lack of sharpness or a cast.
Personally, I even had to put some high-speed films (400 - 1600 ASA) through the X-ray machines used for hand luggage - with no ill effects. (Except for my being apprehensive about those films for the rest of my trip!)
It seems the new, really "dangerous" machines are quite expensive and mainly used on very modern airports on the main baggage.
Still, I always ask for a hand-check, and quite often, airport staff are prepared to take the extra effort.
It's highly recommended to be at the check-in a bit earlier than usual - you are not in a good position to discuss a hand check with airport security while listening to the last call for your flight!
By the way, lead "film-shield" bags will not really help: if they actually do their job, airport staff will just see a black blob within your luggage. The normal reaction will just be to increase the radiation dose until details become visible.
However, I read in a German photo mag that there now is a system that uses psychology rather than lead to protect your film material from security personal. The new bags (manufactured by California Sunbounce) carry a big yellow "Do not X-ray" label and seem to be a good way to organise your films.
Any personal experiences?
PS: X-rays shouldn’t be a problem for a digital camera. However, I heard about their storage media getting wiped out by the strong magnetic fields used to detect metallic objects carried by the passengers. Presumably, it’s best to put your digital camera inside the hand luggage and rather get it checked by the X-ray machine...