Update, with some images of preliminary plans:
http://www.flightglobal.com/blogs/ai...pansion-plans/

The prior plan has evolved a bit. Now, the security checkpoints will be moved to an addition on the mezzanine level (the level where the bridges/moving walkways to the Metro and garage are), suspended above the arrivals roadway. The upper level will remain an outside-security ticketing hall. National Hall will now be entirely "sterile," making inter-concourse transfers much easier. This also doesn't require the numerous vertical transfers previously contemplated.

Floor 4: Ticketing (departures roadway)
Floor 3: Security mezzanine ("tunnel" to garages & metro) [sterile side: lounges]
Floor 2: National Hall, concourses/gates/shops
Floor 1: Baggage claim (arrivals roadway)

Banning the public from National Hall is a bit of a mixed blessing. There have been attempts to market its shops to the public, but they've never been entirely successful (I've sometimes stopped in for coffee while out biking the Mount Vernon Trail, but that's rare), and the increased transferring passenger volume would more than make up for the lost business. Other hubs, like MSP, seem to manage just fine with all of their shops beyond security. This also hopefully will ease overcrowding in the piers -- cleared passengers will not be confined there but can roam the entire second floor.