The McCain campaign was ready to go with an ad about Biden (and I hear several other VP contenders). Their piece on Biden is already on the web, and the 30-second format suggests it will also play on TV during next week's convention. It's simple and devastating.
Every Republican operative I've talked to for the last week thought Biden was the weakest pick. He's a loose cannon who has already made several gaffes in the Hall of Fame of political screw-ups that will now be rehashed for the next week. Obama needed somebody that was reliable and could potentially deliver a red state. Biden is neither. Obama may think Biden shores up his own weakness on foreign policy, but it just emphasizes it. And given that most Americans think economic issues are more important, it's less clear what Biden delivers.
Let me also say that this announcement was a political catastrophe for the Obama campaign. I mean, what kind of presidential campaign announces their VP pick at 1:00 A.M. on a Friday night? This news should have been made earlier in the week -- at the very least, earlier in the day. The Obama communications shop took what should have been an exciting development and buried it in the middle of the night. Sure, the public will still hear plenty about Biden in the coming week. VP picks make the kind of news that can't be buried all that effectively. But this is the second time in recent weeks that the Obama campaign has demonstrated a stunning ignorance of political communications. Don't they know you're supposed to release good news on a Monday morning and bad news on a Friday afternoon?