I'm not sure exactly what the FDA's methods are when they calculate acceptable dose. I know that all of our machines have to be approved by the FDA and dose is just one part of the equation. That said, the occasional healthy traveler probably has little need to worry.
Here are the FDA's procedures for security systems: http://www.fda.gov/Radiation-Emittin.../ucm227201.htm
they don't give actual exposure numbers, but they claim safety is not an issue basically.
Yup...I agree with you about the pat downs wholeheartedly. I fly for company business, and I actually have a trip coming up in a couple of weeks. I have admit though that I would certainly rather be scanned than "patted down" and that's probably what I will do when I go through the security line because frankly making a scene at an airport is nothing more than a good way to get yourself arrested.