They're President for an hour regardless of whether they take some silly oath.
Imagine if this had happened.
11:40am: Bush pardons Cheney
11:45am: Bush resigns
11:50am: Cheney sworn in
11:55am: Cheney pardons Bush
In a scenario where Bush would have to pardon Cheney for something, Cheney would have already been impeached and found guilty by Congress, thereafter he would be removed from office. If Bush were to then resign (and I'm assuming this is 2009 right before Obama takes the oath), Pelosi would become Acting President for the next 15 minutes. Pelosi could then pardon Bush if she wanted to, but would probably be too preoccupied with Steny Hoyer trying to steal her speakership while she was distracted with the succession. Obama would then take office and everyone would be very confused.
