Now don't anyone flame me for this.
Northwest could eventually but not in the next five years because they are ordering more 757s inc the -300 and the 747-400.
The 727s are being replaced by the A320 and I think the A319 will replace some DC-9 routes as it already has been. I kind of think that the A318 will replace the smaller DC-9s. I doubt that Northwest will go for the 737 or the 717 because they will want fleet commonality and stick with Airbus for the small end of the fleet. Don't forget, Airbus has plans and models for the A316 and A317 also, but that doesn't mean they will fly.
If Northwest is really satisfied by the A330, they may replace the old 747-200s with A340s. Eventually (but not in the next 5 years) you may see the A380 replace the 744.
For the 757s? If Airbus would develop what could be known as the A322, that might a possible candidate.
USAirways was probably going to be the first U.S. airline using an all Airbus fleet of MORE than one type (Jet Blue only has 1 type- A320, so no one correct me on that).
But I doubt any of the other US majors will become all airbus, other than Northwest who has the best chance of doing so in perhaps the next ten years or so, but not anytime soon. It all depends on how much they like the A330s, that will make the difference.