Who is to say that Mercedes wanted him at "Brawn" anyway? They will still be very keen to have an all German line-up no matter what they may say publically and aside from that, there are enough doubts about the competitiveness off next years motor at Brawn.
Don't think it's a bad move at all for Button, he's getting twice his wages (still very cheap for a world champ though) and driving a potentially much better car at a team, however it is set up, evidently wanted him.
I think he'll get his arse felt for him but he's earned a shot at a "big team".