Utah has long been rumored to join the PAC10. Actually, the rumor was always Utah and BYU.
I think they look at program quality and program size coupled with national interest and location, but when the final decision is being mulled over in board meetings, academics rise to the top and can influence the final decision.
I guess in Marshall's situation, if Herd football wins them all this year and goes to a BCS game and wins it. Then the following year wins them all again and wins another BCS game, if there is a conference shuffle, MU would be on the table for invite. Then when a final decision is made, the presidents look at MU's academic standings.
Football got them in the door, academics either help them fully in or prevent that invitation.
Which outweighs the other? Prominent football program or so-so academic standing.