I think the problem with Boise St. is that they are small potatoes in terms of size, tv market, sports other than football, etc.
UNLV is another school that comes to mind, particularly with the tv market and all but their athletic programs haven't been headliners in a long time and there's always the gambling taint involved (fair or not) when you discuss Vegas. San Diego St. is in a similar boat without the gambling tie. Fresno St. is another outside chance.
I think if the Pac-10 makes the change, they'll want some splash and the athletic programs and academic programs at Utah and Colorado make for sufficient splash, particularly when the Salt Lake City and Denver tv markets are added to the mix. CU is on the downside athletically but they'll eventually bounce back and the PAC-10 gives them better access to the California kids they like to recruit.