If Boise State and TCU stay undefeated, and both Auburn and Oregon stumble and lose a game, a NC game with Boise State and TCU is not only possible, it's quite likely to happen.
It's impossible to predict how the Harris Poll and Coaches Poll would cast their ballots in that situation but if you look at the point totals in the latest polls, the 4 undefeated teams have huge leads over the highest ranked one loss team. (#5 Wisconsin in both polls)
TCU and Boise State are already getting about as many #1 votes as Auburn. And so far this year, no one loss team has received any #1 votes in either poll.
But who knows what the voters will do. They could collectively stay with their established pattern or they could yield to their biases and keep Oregon or Auburn at #2 ahead of the one or both of non-BCS conference schools. If Oregon or Auburn lose a game because of a fluke play or bad referee call, keeping that team at #2 could be justified.
The computer 1/3 of the BCS is easier to predict. The two major components of all 6 computer ranking systems are win/loss records and strength of schedule. A loss to #12 Alabama would help Auburn's SOS some, but the loss itself would drop them down below TCU and probably Boise State in the computer rankings. TCU is already ranked #2 by the computers.
So it would depend on how the poll voters shift their support and how the math works out when the computer 1/3 is averaged into the result. But with the scenario you suggest, I'm guessing we'd see a TCU vs. Boise State NC game.
Yes, I would enjoy a TCU vs. Boise State NC game. I think we're in for a treat with any match ups involving those 4 awesome teams. It should be an exciting finish to a great college football season.