Jason is correct that the matchups are based on pre-season expectations and not on a strict order-of-finish pairings.
However, since the ACC expanded to 12 teams in 2005-06 (requiring one ACC team to be left out) the last place regular season finisher has been left out of the challenge the following year and 11 games have been played.
ACC-Big Ten Challenge results.
Wake 2005-06
Miami 2006-07
NC State 2007-08
Georgia Tech 2008-09
I don't know if there is a definite rule about this, but since GT was picked to be in the top 3 or 4 ACC teams in the preseason and was left out of this year's challenge, it seems to me that there might be.
Of course, if there is such a rule I am sure it will be changed if UNC finishes last this season.