Details about Pre-Check at IAD.
http://pizzainmotion.com/2012/09/25/...ing-at-dulles/
It seems that if you get rejected from Pre-Check, you get sent to a WTMD-only lane, so that is good news. And I guess if you don't have Pre-Check, you can still walk up, get denied, and go through the 'regular' WTMD-only lane.