Denver, Texas San-Antonio, and Texas State have all officially received and accepted invites from the WAC, with membership effective 2012. 

From a WAC release:

The announcement was made by the chair of the WAC Board of Directors and Utah State University President Stan Albrecht and WAC Commissioner Karl Benson after the Board unanimously voted to issue the invitations earlier this week.

“The Board of Directors is extremely dedicated to the long-term stability and growth of the Western Athletic Conference," said Albrecht. "The University of Texas at San Antonio, Texas State University and the University of Denver are three very strong institutions, both in their commitment to building athletic excellence and in their commitment to academics. We believe that all three will help us in our quest to make the WAC one of the outstanding athletic conferences in the country.”

Although there is no official word on Seattle University’s status, Montana’s decision to stay in the Big Sky is a significant development that opens up that possibility.