Based on game winning probabilities that can be found using my college football matchup analyzer and which, in turn, is based on my college football ratings.
My system's undefeated probabilities (takes into account probability of playing Nebraska vs. probability of playing KSU):
Texas: 67.71%
Florida: 42.69%
Alabama: 39.88%
(Either SEC Team: 82.57%)
TCU: 98.76%
Cincinnati: 46.21%
Boise St.: 73.61%
Odds 5 teams undefeated: 18.78%
Odds 4 teams undefeated: 41.75%
Odds 3 teams undefeated: 29.92%
Odds 2 teams undefeated: 8.64%
Odds 1 team undefeated: 0.89%
Odds 0 teams undefeated: 0.01%
Odds of reaching title game as undefeated team if no BCS leaps occur:
Texas (win and in): 67.71%
Florida (win and in): 42.69%
Alabama (win and in): 39.88%
TCU (needs to win plus zero or one of the above 3 undefeated): 43.55%
Cincinnati (needs to win plus zero or one of the above 4 undefeated): 2.82%
Boise St. (needs to win plus zero or one of the above 5 undefeated): 2.44%
The last grouping doesn't add up to two because of the slim chance of a one-loss team in the title game. I'm assuming, of course, that any loser drops below all the undefeateds in the BCS rankings. That's extremely doubtful in the case of an SEC title game loser dropping below Boise, obviously, but it made the math possible.
Keep in mind that injuries (e.g., Christian Ponder) and other extra information are not taken into account.