Microsoft has agreed only to hold your personal information (read: darkest secrets] for six months, while Google and Yahoo will continue to keep them much longer to "improve search quality." By which, of course, they mean "ad revenue."
Granted, the Bing move comes after pressure from the European Union, who's fined Microsoft so many billions of dollars in the past that the latter is understandably twitchy. It's still a policy that should be applauded, though, to the same extent that Yahoo and Google's lack of improvement should be roundly booed. Google stores cookies for a year and a half, far longer than they should need it for any purpose other than sending you the perfectly targeted Bowflex ad.