Use the custom GA tracking for campaigns to do it, creating a special campaign for paid bing traffic.
Have a look at this article: http://www.practicalecommerce.com/articles/1288-Integrating-Yahoo-Search-and-Microsoft-Bing-with-Google-Analytics - this provides a good solution.
The article also mentions that you can use the Bings adCenter Desktop tool ( http://advertising.microsoft.com/support-center/adcenter-downloads/adcenter-desktop ) to setup the campaings.