I too wouldn't bother with lead bags. Film of iso800 and under will hardly be affected in the first place, even after many passes throught he carry-on luggage X-ray machines (I've had iso800 film go through the machine about 10 times on a trip to and from Beijing. When I finally used it, it showed no fogging whatsoever). And film of iso1600 and up can be hand inspected. Even if they don't jack up the intesity of the X-ray machine, they're at least going to ask you to open your bag anyway as the lead is shielding the contents of the bag from being seen by the operators.