Senator Obama has chosen a “mainstream” Democrat, which is meant to re-assure his base that he is not a ‘wild-eyed’ crypto-Marxist revolutionary.
The narrative by the Mainstream Media will be his foreign policy experience (committee chairman?) and defense experience (Huh?). From the point of view of the Democrats, Biden adds a lot to the ticket. But I am not sure how well this will appeal to the ‘undecideds’. And it takes away from the Democrats ability to attack McCain on the age issue.
He knows McCain. He will attack McCain. McCain will respond. McCain will attack the #2 position on the ticket, and not Obama, as intended by the Democrats. This is basic modern presidential political manuevers.

McCain must link all his future criticism of what Biden says about him, to Obama.