Several of my sites never ranked at all in Bing until recently. It seemed like they had penalties of some sort but since they ranked well in Yahoo and Google I didn't want to change them. So I never bothered to try to figure out why they received so little Bing traffic.
Now more than one site has gone from either not listed at all in Bing or maybe 2 or 3 Bing referrals a day to first page and even sometimes #1 ranking for their keywords - just like they rank in Yahoo and Google. Almost overnight they started getting search referrals similar to Yahoo and Google, proportionate to Bing's market share.
I'm glad for the sudden changes but I wonder what happened? Is Bing trying to align their results more closely with Google results?
I hope all of the rankings don't disappear again as fast as they showed up.