From the USPS FAQ:
Is Mail X-Rayed?Some of the mail that is sent through the United States Postal Service will pass through an X-Ray machine. There are no specific guidelines as to what may or may not be x-rayed, though mail sent to or through larger cities is likely to pass through an X-Ray machine.
Am I being paranoid to think that this could be a non-trivial issue for someone mailing undeveloped film?
