This is very very early and I don't have evidence of this myself, since I don't monitor rankings, but a single post at WebmasterWorld suggested there is a Bing update taking place.
textex, someone I trust at the forum, who has been a member of WebmasterWorld for almost ten years now, said:
Looks like an update....
So if there is a Bing update, there is a Yahoo update in the US and Canada.
Did you notice major changes in your Bing rankings?
Forum discussion at WebmasterWorld.