Ill believe it, when Hamilton is wearing a deep purple/dark blue overalls with a giant RedBull logo on it. If he is in talks then its a risque move for him, McLaren are hardly that far from Rebull, if he was drvinig for Williams now and not McLaren it would make more sense. These things tend to go in cycles and although Redbull have at or near the top for th past 2/3 years it doesn't mean they will be by 2013, so much can happen, RBR could simply lose form McLaren could find that missing link in the enginerring/desgin department. Newey could retire from F1 to his boats, and leave RBR in the in the lurch a little, he seems to condsider it every couple of years and who knows it he might actually do it and it wouldn't be easy to replace him.
Playing Devils advocate a little, If today Hamilton announces a move to RBR for 2013, would be out of blind luck of things working out, or an educated guess? I suspect the former.
If it was me, and lets face it what do i know, If i was looking to change to a better team. I would look at who has the best infostructure for next season and not try and predict what it will be a couple of years time.