The BCS #1 and BCS #2 always play in the BCS title game. When the BCS was just the Bowl Alliance, the Big 10 and Pac 10 weren't part of the system.
Right - but I think this was the condition for Big 10/Pac 10/Rose Bowl to join the BCS. Something everyone wanted by the way. Before the Big Ten/Pac 10 joined they still had to meet the Rose Bowl commitment - which is why Michigan was screwed by the polsters when they took away their trophy (after giving it to them on national TV) and gave it to Nebraska the last year before the Big 10/Pac 10/Rose Bowl joined the BCS.
I think that the only way the Rose Bowl was going to let the two conferences out of their deal was if a #1/#2 Big 10/Pac 10 match up occurred they met in the Rose Bowl.