I think Cal can play in the BCS Championship game if they have a BETTER record than all but one of the following - (Texas, Oklahoma), (Florida, Georgia, Alabama, LSU), (Va Tech, Florida State), Notre Dame, (Penn State, Ohio State) and USC* (given that they beat USC).
I'm not sure I'd go much lower than 20-1 on the above happening. When you factor in beating USC and actually winning the championship game, it looks like a lot longer odds to me than 51-1, so personally, I'd have to pass.
* the reason schools like Ole Miss, Oklahoma State and Georgia Tech aren't in the list is that I don't think that they would play in the game even with an equivalent record due to lack of a current big-time reputation and the prejudices of the voters (see Auburn '04).