The DC travel forum has a thread on the subject.
http://www.flyertalk.com/forum/washi...airport-2.html
Suffice to say, a new C/D terminal is a mere gleam in the eye. There's no funding for it right now, and just general concept plans, nothing specific.
There was an article in the Post yesterday (I think) about building a new road to Dulles to expand its cargo capacity, noting that business there (cargo + pax) has been flat or declining for the last 5 years. Given that, a new terminal isn't likely to seem urgent.