My take on the C7 expansion
The reason they are starting off with 10 teams and moving to 12 is that part of the deal negotiated during the breakup of the Big East was that Louisville, Rutgers and Notre Dame would also leave in 2013 (at least for Basketball) and possibly play 1 year with the C7 before departing for the ACC and BIG.
I think the reason no announcement has been made is due to ongoing negotiations with the ACC/BIG regarding Louisville, Rutgers, Maryland, and Notre Dame joining their respective new conferences in Basketball for 2013. With the most likely scenario that Notre Dame is allowed to join the ACC early. Maryland stays put in the ACC till 2014. Rutgers and Louisville play 1 year of football with the old Big East and 1 year of basketball with the C7 and 3 additional teams. Butler and Xaxier will take the first two spots and the last spot for 2013 will go to either SLU, Creighton. In 2014 Louisville and Rutgers will be replaced by 2 additional teams, one from the midwest (SLU or Creighton) and one from the east probably Richmond.
I still think Dayton gets left out.