Obama already has an ad mocking the endorsement that you can see here.
And here's Obama's statement:
"I'd like to congratulate Senator McCain on this endorsement because he really earned it. That endorsement didn't come easy. Senator McCain had to vote 90 per cent of the time with George Bush and Dick Cheney to get it. He served as Washington's biggest cheerleader for going to war in Iraq, and supports economic policies that are no different from the last eight years. So Senator McCain worked hard to get Dick Cheney's support."