Exact same thing happened on Yahoo search results (yahoo.com) as did with Google and their Penguin/Panda thing... dropped completely from 1st two pages on my two highest keywords. Just within the last day or so. So far, the listings/ranking have not changed on ca.yahoo.com though.
I think it is old news that yahoo/bing copies google results. I have read google pulled a trick on bing by adding some bogus entries to google search results and bing have promptly copied them over!
cujo: What type of SEO have you been doing? Have you been adding fresh content, creating backlinks, etc.?