According to this article from Wired last year, TSA changed the rules so that now you can only fly without an ID if you claim that you lost it. So essentially they're giving up any increased security advantage (real or imagined) that comes from a mandatory ID requirement, for the purpose of sending the message that challenging authority will not be allowed.
http://www.wired.com/threatlevel/2008/06/tsa-nixes-flyin/