Basics: there are four BCS at-large spots available. They can go to any team in the top 14 of the final BCS standings. The BCS bowl selection order is: #1 team's "home" bowl, #2 team's "home" bowl, Fiesta, Sugar, Orange.
As things stand now, the SEC and Big XII are virtual locks for at-large BCS births. A non-AQ school earning a spot is highly likely, so that leaves 1 open spot. That one spot is most likely to fall to a team from the Pac-12, Big Ten, ACC, or Notre Dame. If Notre Dame runs the table and goes 10-2, they will be the pick. An important note to remember: the only team to ever earn a BCS at-large slot after losing twice after November 1 was Iowa in '09. The Big Ten needs Notre Dame to lose at least one more game. Here are some scenarios that would give the Big Ten a big advantage (for the sake of my scenarios, I'm assuming the Big Ten holds an at-large advantage over the ACC and the Big East):
1. Wisconsin loses in the Big Ten title game to fall to 12-1. A 12-1 Big Ten team would be very difficult to pass up.
2. Oregon loses to Stanford and one other Pac-12 opponent. This would leave the #2 Pac-12 team at 9-3. A 9-3 Pac-12 #2 option would likely get passed up in favor of a 9-3, 9-4, or 10-3 Big Ten #2 option.
3. Oklahoma State loses to Kansas State or Baylor and loses to Oklahoma. This would severely hurt their odds of finishing in the final 4 and would make it so an extra BCS slot would open up. A 10-2 Oklahoma State team coming off a pair of late season losses wouldn't be an attractive Big 12 #2 option.
4. Oklahoma State beats Oklahoma in Bedlam and finishes #2 behind an SEC team in the final BCS standings; Nebraska loses to Wisconsin in the Big Ten title game but still finishes in the top-14 in the final BCS standings. In this scenario, the Sugar Bowl would replace Bama/LSU with LSU/Bama and the Fiesta Bowl would have the next two selections. An OU-Nebraska Fiesta Bowl matchup would be utterly irresistible for bowl officials.
5. West Virginia wins the Big East; Stanford wins the Pac-12; Alabama and the Big XII champion meet in the title game; Sugar and Fiesta replace with LSU and Big XII #2; and the Fiesta Bowl picks Boise State third. This would put the Sugar Bowl in a tough spot. They would already have LSU, so picking WVU or Oregon would cause a regular season rematch. They could do this and let the BCS and conference officials move every one around (which is technically a part of the rules), or they could pick a Big Ten team.
While #4 might be fun, I think #5 is the most likely scenario, but in order for it to happen, Big Ten fans need to root hard for Alabama to beat LSU. Here are my guesses at what the BCS bowls will look like:
Title game: Alabama-Oklahoma
Sugar: LSU-Nebraska
Orange: Clemson-West Virginia
Fiesta: Oklahoma State-Boise State
Rose: Wisconsin-Stanford