The Democrats could have nominated Robert Byrd in past years, dressed him in his Grand Kleagle gowns, and they still would have won 90% of the black vote. This would be a move more likely to appeal to the soccer moms and Volvo Republicans.
I doubt Powell would do it, though. He has a better life on corporate boards and making speeches than he would as VP. He did give McCain $2300 so I don't think he'd be Obama's VP.
I think it was Bob Novak who wrote recently that both McCain and Obama were seeking Powell's endorsement.
Powell, having a high opinion of himself, likes to be stroked.
Consider this "rumor" part of the stroking.