I don't trust the airport screening systems despite what they say. I just made a photo expedition to San Francisco. Shipped the film to the hotel by fedex and had a return shipping label to ship it back. Hotels ship stuff for customers all the time so it's pretty routine for them. I also put those Kodak DO-NOT-X-RAY labels all over the box.
Anyway, everything went very smoothly, I've had film hand checked at the airport, but they usually try to talk me out of it, so they clearly don't like doing it. When I left SF, the line for the security screening was very long, so I'm glad I didn't have to worry about it.