I had a chance to stop by IAD today, and I found the upcoming PreCheck checkpoint to be quite substantial. The checkpoint is still under construction, but is mostly complete. It is located in the center of the departures level of the Main Terminal, near the Harry's Tap Room. Four baggage screening lines are present, along with three WTMDs, and space for a fourth. There is no AIT in sight. Farther east on that level, between check-in zones 3 and 4, is an information desk about GE and PreCheck. One of the TSOs working at the desk said that the checkpoint would open on 9/25.
We have yet to see how the checkpoint will be staffed and operated, but, from what I have seen today, I must say well done to the TSA and MWAA.