The Experian Hitwise data for January [hitwise.com] shows some interesting gains for Bing and losses for Google: Bing gained 6%, but Google lost 2% - and Yahoo lost 4%.
More interesting to me is their data on "success rate". Here there is no month-over-month change for any of them. However, the baseline shows a major difference. Success rates on Bing are measured at 81.5% and at Google, 65.6%
On reason for this is probably the definition of success. The detail isn't made very clear, but it does involve visiting another website. As we've been observing for quite a while, Google SERPs contain a lot of information on their own, and the user often doesn't need to visit another website to have found what they were looking for.