http://www.flyertalk.com/forum/5892196-post22.html

Two years ago I did the same thing but in reverse— went to BWI instead of IAD. Oddly enough, I actually got better flights both times— upgraded from a middle seat on the first incident, and a more direct routing on the second that actually got me to the destination earlier than the original itin— on Thanksgiving!

Both times I got stamped through security with nary a batted eyelid. The gate agent couldn't take care of me at the gate during the second incident, and had me go out to the ticket counter to get rebooked. The TSA ID checker recognized me and jokingly asked whether I'd flown around the world already. He was not so loud after I quietly explained that he had let me through with an invalid boarding pass….