A couple of updates:
As of 9/6/2005 United is moving from the D terminal (mostly YX) to the C terminal. It is also rumored that US Air will move from D to C by the end of the year (America West has their gates in C) thus leaving Midwest the only carrier out of D.
When I passed though security for the E Concourse before my NW flight on Wednesday, work had begun to widen the TSA checkpoint area.
According to the website a renovation of the D concourse will begin later this year.