Okay, there is NO WAY Dumbledore is evil, or in league with Voldy, or any of that stuff. I know JKR likes to trick us, but some things are sacred. I agree with Zora, I think Dumbledore realizes that Voldy's done something that will ultimately backfire on him. Think of this good vs. evil struggle as a chess match. V's just made a move that looks good in the short term, but D's spotted the mistake that may lay him open for defeat many moves farther into the game.
BTW, this has been a VERY hot topic on some of the other HP groups.