It's hard to guess what is in the mind of The Committee at FOX.
My best guess is that they expect that Obama would not have accepted if he were not ready for the "regular questions", the ones that everyone has been throwing in the past weeks.
Sensing also that the Reverend Wright story is about to blow-back up their noses, they are going to want to break new ground.
I'm sensing that they are going to play the abortion card, as strong as they can nail him on it, and a bunch of constitutional law or Terry-Shiavo-like "law" cases. I'm betting, at this stage, that Roger is still hoping to fire up the base, who are moving ahead without McCain, even, this cycle (LOL - it's true).
Wallace really, really likes National Security questions. They will press Obama on any AIPAC-related concerns, would be my guess, because McCain really, really likes the support he got in Florida and the GOP really likes Freedom's Watch dot org.
Update:
One real test will be whether they go after immigration. As many have remarked, that could be this cycle's "hot button" issue. My guess is still that they will go with abortion first, and bank the immigration questions.