Per ESPN: Boise State has joined the Mountain West. The story is still developing and I'll update this later today.
UPDATE: First of all, a link. Second of all, this move was pretty much automatic once Colorado went to the Pac 10. Assuming that the MWC has no real interest in Baylor, they'll only be competing with the Big East for the Big 12 refugees after this phase of expansion. And Boise State isn't taking a spot that another school could theoretically fill.