@Cycloptichorn,
I think he's just flustered.
Honestly.
I think he doesn't like how things are going and was getting nervous about the debate. I think that came first, and then a "hey we can show that we're serious about this whole economy thing by 'focusing' on it -- yeah, that's the ticket!" justification followed.
I think it's also an attempt to take control a bit, put Obama into reactive mode. Does he (Obama) say "You have a point there Johnny, let's focus on the economy" and then McCain gets credit? Does Obama say "heck no, get back here and debate" and then he looks petty?
I think that's the calculation. And I do think Obama has to be careful, I think missteps can be made.
But... daaaaang.