While working at Starbucks, I can't tell you the number of times someone would ask me this question! Or, the number of times people would be shocked when I kindly told them that yes, their "decaf" coffee does actually still have caffeine in it. If you want to go caffeine free, you're going to have to hit an herbal tea.
This video gives a really simple explanation of how decaf coffee is made. If I could show this to every decaf, no-foam, no-fat latte customer who "can't handle any caffeine," trust me, I would have.