I don't get why any top-line driver (and a World Champion is surely top-line no matter what your opinion of him) would go to McLaren and play second fiddle to the Lewis Hamilton Show.
It would indeed be a VERY bad move for Button.
The problem being that Hamilton is settled at McLaren and will therefore have a strong advantage. Effectively, he is on home turf. With this in mind, unless Button places bombs in Hamilton's car, during 2010, he will lose to Hamilton, by a large margin.
I do believe Hamilton is faster, however, the "home" advantage that Hamilton will enjoy, will magnify his superiority and make Button look worse than he actually is.
The only reason for Button to move to McLaren, is if he has a very big ego (like Senna), has absolute belief in his own ability (which he doesn't) and now wants some credibility, within F1 and wants to go head to head with Hamilton.
I think if Button joins McLaren, then like in 2008, it will be Massa and Alonso VS Hamilton (with the other McLaren car competely AWOL). If Kimi goes to McLaren, I do believe he will give Hamilton better (unintentional) support and at least fight directly with Massa.
If Kimi goes to McLaren, next year should be a titanic battle: Kimi and Hamilton vs Massa and Alonso.
Button to McLaren would be a bad move for McLaren, Button and F1 in general.