ND loves playing the hot/cold game with the Big Ten and vice versa. When BCS conferences start going beyond 12 members, ND will be "forced" to join a conference for football or risk becoming irrelevent. They can play "hard to get" all they want, but they know when they're choosing between the Big East, ACC, and Big Ten for whom to align, the Big Ten is the best fit for them and has the most to offer them to improve them as a school: academically, athletically, and monetarily.
I don't think the doors well ever be completely closed to ND joining the Big Ten, but if they do close, it will be because ND thinks they are above joining a conference. In the meantime, the Big Ten will give them every chance to join. The big question is when the Big Ten finally gets to 15 teams, what will ND do when they finally get that final ultimatum?