Just kidding. It's actually his way of congratulating John McCain on his campaign:
"But it's just an accident! An isolated incident!", you screech. "He's not really a sullen child who lashes out passive-aggressively when somebody stands in his way!" Well, he's certainly racked up quite a history of such "accidental isolated incidents" in a short period of time. This fits right into his established pattern of behavior. Just ask the PUMAs if they think it's an accident when Obama's middle finger comes out to punctuate his point about a rival:
Dear Obamabots:Posted by Jim Treacher at November 3, 2008 11:30 AM
If you want us to stop "distracting people from the real issues" -- even though you also complain when we bring up real issues like Obama's tax plan, his economic theories, where he gets his ideas, where he gets his money, where he gets his votes, etc. -- you might want to convince your hero to stop flipping people off in public.