I guess I should have added a rundown of the speculation that I've already seen:

In PHL, it appears that CO would keep its gates in the D concourse (D6/D8), unless CO and UA/AC all move. Given that UA has a good presence in D, this would work for the shared ground handling that has been discussed.

There is a thread discussing moves in SEA that inspired me to create this clearinghouse of a thread. UA, US, DL, WN, and CO are all mentioned in there as candidates to move based on actions that AS may or may not want to take.

The Box had a healthy ATL discussion a while back, and two theories were advanced, both of which involving CO and the PC moving from Concourse D.

If anyone wants to add to these or bring their $.02 in from where they're based, let's do it.