I think this has a lot more to do with Flip Saunders than people think, because obviously as said, this isn't about the money. Detroit offered Ben $48 million over 4-years and he called the proposal disappointing, yet he signed for not much more. I think it's a matter of not believing in Flip, then looking at an even better situation long-term in Chicago.
Think about this for a minute: Detroit falls short again next year in the playoffs with Big Ben under contract for the next 3-years, and Chauncey Billups opts out of his deal. Ben would then be stuck for 3-years of not having a chance to compete with the top teams in the Conference, much less being one of the top teams. I think Detroit now goes after Joel Przybilla.
For Chicago, I still don't see them as a "true" Championship caliber team. I think they'll contend, but I don't think they have enough to win it all. They don't have a post player who can score, nor do they have a perimeter player who can take the ball to the basket. Chicago still relies on shooting, and that doesn't win Championships in the NBA, I'm sorry. Might I add, Tyrus Thomas & Ben Wallace compliments each other in horrible fashion. This signing now makes you question that selection even more.