It was supposed to be 4 countries (Ind, Aus, S. Africa and England) and top 2 T20 teams from these countires. England however were allowed to have one team only, Middlesex. Their runners-up, Kent were not allowed to partcipate since players like Azhar Mehmood, Justin Kemp etc are still playing for Kent as well as ICL and IPL authorities are pretty allergic to anything related to ICL.
Therefore they needed the 8th team and picked Pakistan. As a result the RBS T20 (our local T20 in October) became more interesting as the winner was provided an opportunity to feature as the eight side for the Champion league T20.
Next year they might increase the number of teams and will invite teams from Windies and Lanka.