A lot of the legwork was done by @BobcatReport, a Texas State fan site. They discovered this agenda for the state university system of Texas's board of regents meeting. As you can see on the first page, the last item on the agenda is "U.T. Arlington: Approval of change in athletic conference membership to join the Western Athletic Conference." The Bobcat Report also confirmed with the Southland Conference, UTA's current home, that the Mavericks will be joining the WAC.
One caveat: UTA has not played football since 1985. Karl Benson's assumed plan is to reach nine football-playing members, and 12 members total. The current move brings the WAC to ten total members, seven of which play football.
WESTERN ATHLETIC CONFERENCE (July 1, 2012)
To fill out the assumed nine-football-twelve-total plan, the WAC simply needs to add two more football-playing schools. Perhaps another Texas invitation in Arlington shows that the WAC may be considering Lamar after all. Or, Benson may still be efforting to bring in California schools like UC-Davis, Cal Poly, or Sacramento State; or, is perhaps still trying to sway Montana and Montana State away from the Big Sky.
If nine-football-twelve-total is the plan, the invitation to UT-Arlington shuts out any further invitations for schools such as Utah Valley and Cal State Bakersfield who also do not play football.