This is just a guess but I think Barack Obama will announce his V.P. pick this afternoon. To maximize the announcement, I believe he will "text" his choice 15 minutes before the evening news (eastern). If he doesn't do it by then, there is a lot of dead time when people will be doing other things between today and the time when he is supposed to be on the steps of the old Illinois Capitol Building; the place where he launched his campaign for the presidency. Obama is all for maximizing the show. I believe he will, once again, try and maximize this by forcing the Evening News to abruptly focus on him.
Just my opinion.