Man, there's some potential for hilarity there if that works. I have one of the older SSN cards, which says clearly along the bottom that it "shall not be used for identification purposes" (I'm given to understand that more-recent cards don't say that anymore). It'd be worth every minute of hassling I'd get if I ever used it, had it accepted, and then asked the TSO why he would accept a card for ID purposes which clearly states on it's face that it CAN'T be used for that.

Of course, it's not a photo ID, so it'd probably flunk on that anyways.