At PHX there are little boxes with paper surgical booties on stands just before security checkpoints. I always take a pair and use them to walk through security, after dutifully placing my shoes in the plastic bin. It's appalling how filthy the booties are after just shuffling through the metal detectors. I cannot imagine walking on the floor in sox, stockings, or barefooted --- then putting shoes back on. Eeeew.

We have a big box of the booties at home, purchased at a surgical supplies store. (We use them for workmen to cover their shoes when inside our home.) I would encourage frequent flyers to buy their own booties if the airports they fly to/from don't provide them. The cost works out to about 50 cents per pair.