Earlier this year, Bing activated SSL as an opt in option, the only way to see it was to go to https://www.bing.com/ directly. By default, Bing, unlike Yahoo and Google, is not SSL.
But if you search for Bing on Google, you'll get the SSL version of Bing. Try searching on Google for [https://www.google.com/search?q=bing">bing] and you'll see the SSL version.
This was spotted by Kenichi Suzuki first.
The interesting thing is that if you go to Bing, bing.com does not 301 redirect to the SSL version. The rel=canonical actually shows a link to the not secure version also.
So why is Google showing the SSL version?
Maybe because Google wants you to search Bing over SSL, that is if you won't search Google.
Forum discussion at Google+.
