X-rays can fog photographic film. The damage is cumulative, repeated passes will have an effect even on low-sensitivity film that would not suffer too much from a single pass. Most of the films I use are specialized professional emulsions not readily available in ordinary stores (or even in the USA, as with the three rolls of Fuji Natura 1600 I recently ordered from Japan). That is why I travel with film, and on general principle avoid having it passed through X-ray machines by requesting a hand inspection instead.

Grousing about the Transportation Security Administration (TSA) has almost become a national sport. I tend to disagree — TSA has set uniform standards and a measure of courtesy and customer-service orientation compared to the earlier hodgepodge of private contractors. It is much easier to travel with unexposed film in the US today than it used to be post-9/11, pre-TSA. In other countries, screeners will routinely ignore your protests and unceremoniously shove ISO 1600 film into a high-energy scanner, all but ensuring they have fried it.

These common-sense guidelines will ensure speedy processing and avoid aggravation, for yourself as well as for fellow passengers down the line:

• Unpack your film from its cardboard boxes and plastic canisters (or foil wrappers for 120 film), and put it in a transparent zip-lock bag (the ones with the easy-open plastic sliders are best).
• If you are (justifiably) afraid that film outside light-tight canisters may be fogged over time, carry your canisters separately (J&C photo sells inexpensive plastic canisters for 120 film, or you could splurge for the aluminum ones by Acratech). Probably a better option is to use a thick black plastic bag like the ones used to pack photographic paper.
• Pack at least a couple of ISO 1600 or higher rolls of film so you can ask for hand screening. Some TSA personnel will ask you if you have any film higher than ISO 400, having some on hand is simpler than haggling for hand-inspection of low-ISO film. Only once did I have TSA staff separate my higher than ISO 400 rolls for hand screening and pass the rest through the X-ray machine.
• Lead-lined, supposedly X-ray safe film pouches are a waste of money. Not only are they heavy and ungainly, but the X-ray operators will simply increase power when they see an opaque bag that could conceal a weapon or explosives. TSA policy allows you to ask for a hand inspection, just avail yourself of this.
• Pack your film in your carry-on luggage, preferably in an easily accessed compartment

For more authoritative statements, check out the official TSA and ITIP pages.

Digital photographers are not completely off the hook. While ordinary X-ray machines do not affect flash media, the newer high-energy machines currently considered for deployment can alter electronic media. Another interesting aspect is that of cosmic rays, high-energy particles from outer space and which can “flip” bits in a flash card or magnetic media. In a negative, the blip will be imperceptible, but digital files are less tolerant of corrupted bits. IBM has spent quite a bit of R&D effort in quantifying the problem. When you have several billions of bits in RAM or trillions of bits in a hard drive, even an unlikely event like an alpha particle becomes statistically probable, and they have taken special measures against this. Many commonplace materials are also naturally radioactive.

The effect of cosmic rays at ground level is limited, but airplanes flying at 10-25km altitude are much more exposed to them, by a factor of hundred or so — see Dr. Ziegler’s article (PDF) on terrestrial cosmic rays. The impact of cosmic rays on flash and microdrive media is poorly understood today and the risk not fully assessed. Due to short-term financial thinking encouraged by Wall Street, few corporations invest today in the kind of R&D that could conclusively answer this question.