Chitika, a Web advertising company, took a look at Bing usage in the various versions of IE and found a pretty impressive curve: 7.60 percent of older versions of IE search with Bing, 11.00 percent of IE6 users search with Bing, 14.59 percent of IE7 users search with Bing, 16.94 percent of IE8 users search with Bing, and a whopping 22.90 percent of IE9 users search with Bing.
The difference between the last two versions, IE8 and IE9, is the largest one: a 35.18 percent increase. This number may decrease as more users upgrade to IE9: given that the browser is not even a month old, it could be that Microsoft fans are the first to switch, skewing the stats.
It's also worth noting that if a user selects a search provider as the default in a previous Internet Explorer version, that preference will be preserved when they upgrade to IE9. That being said, if they have not picked one, IE9 will use Bing as the default, unless they change it after upgrading.