Have you actually seen a copy of Button's contract then
Why did Brawn say (up until Button was seen at Mclaren) that 2010's drivers hadn't been sorted out yet? Surely they would have confirmed this 3 year contract.
I was about to explain what I think has probably happened (to do with contract options and Button's paycut at the start of the season), but I know you'll just quote that part of my post and say "Have you seen the contract?" so I'll save it for another time.