The more I read and think about this, Texas will NOT join another conference unless it gets a VERY sweet deal - like the ability to have their own network (not going to happen with Pac 10 or Big 10), to select their own puppet commish and dictatorial power for life. If NU/MU leaves they bring in TCU to replace NU and LaLa to replace MU.
Thinking about this a bit more, I think Texas approached the Pac-10 with this proposal. Texas wasn't serious about it, and the Pac-10 probably assumed as much. However, Texas being the gigantic cash cow that it is, the Pac-10 said they would be welcome on the off chance that they were actually serious about it. Texas created this rumor as a bargaining chip against Nebraska. There's a chance that the Pac-10 and Big XII (the "loyal" Big XII members, that is) put this together with the interest of getting a mega TV contract with Fox, but I think Texas is probably just up to their usual shenanigans.
When all is said and done, I wouldn't be surprised if the following happens: the Big Ten will finally make Notre Dame an offer they can't refuse, which puts to bed the whole mega conference thing. NU will stay in the Big XII. Boise State will go to the MWC, and those schools will be one major step closer to an automatic BCS bid. The whole system is still corrupt, though slightly less corrupt than it would be in the Pac-16/Big-16/SEC-enstein world that we find ourselves perilously close to right now (if you think the BCS is corrupt, just wait for what happens when most of the Big East and leftover Big XII teams aren't "eligible" for a "playoff" because they aren't in one of the 3 megaconferences).