While I was IAD a few months ago, I approached the TSA checkpoint with a white T-Shirt and an unbuttoned, untucked dress shirt. The screener asked me to remove my jacket. I said, "well, what if I button it here" and I buttoned a single button in the middle of the shirt (still untucked), and so I said, "now, it's a shirt right?" He seemed a bit befuddled, and all this happend very quickly. The airport was not all busy, and so I pretty much didn't even have to break my stride as all this happened and I went through the WTMD without difficulty.