Regardless of whether or not the moderators give them a chance to make an opening statement or not the candidates always take the time to do a little intro, if necessary by responding to the first question with something like "that's a good question and i'll answer it in just a second, but first i'd like to thank you for inviting me here and giving me the chance to debate the issues with my opponent, and i'd like to thank the audience for showing up to listen to us..." etc.
After finishing the obligatory thanking everyone bit Obama's ought to continue on with something like "I'd really like to talk about the important economic issues facing the American people today, but [turning to McCain] I would like to know John, are you going to insist on discussing the lies spread by your ads and by Governor Palin, or are you willing to dismiss them as the lies they are so we can discuss the issues that really matter to the public?"
He could say it better than me i'm sure, but something direct and confrontational at the beginning, stating that he wants to discuss the economy and forcing McCain to choose between diverting the discussion or giving up the Ayers and such attacks. If he lets McCain bring the issue up at some later point of his own choosing McCain _might_ be able to work around to it without actually looking like he's trying to sideline the entire debate.
So force McCain to choose up front, and either acknowledge that he's avoiding the important topics, or discredit any attempts to bring it up later. And if McCain _does_ bring it up, either at the start or later on, then Obama should respond quickly and in series with the "i was 8 at the time" bit, and ask him about his relationships with Keating during the savings and loan scandal and why he hired an advocate for Saddam Husein as a campaign adviser and why Palin is lying about the findings of the investigation into her ethics, but then follow that up with "but these aren't the issues the American people want to hear about, they want to hear about how the economy is going to be fixed." He should always respond to the attack in force but then try to get the topic back onto the economy, so McCain gets all the "credit" for changing the subject.