Hypothetically, if the Texas legislature pushes for Baylor to be taken over Colorado into the Pac-10, I could see the Mountain West looking to expand with Boise, Kansas, KSU, and Colorado.
Or really, in any circumstances, they're probably waiting to see who is left over knowing they'll have a good chance picking up the scraps left behind by the Big 12 implosion, whatever scraps that may be if the implosion comes.
If the MWC doesn't get Big 12 scraps though, they need to go after Fresno State, Boise, and Nevada out of the WAC first, and past that I don't know who.
ETA: As Robbie said, if the BCS keeps 6 conferences and the Big 12 has imploded, the MWC would likely snatch up that auto-bid if they were able to take the top table scraps from the Big 12.