Yahoo and Microsoft's search brotherhood is finally going to bear fruit as of this week when the back-end technology for Yahoo! Search in the US and Canada (English) will begin to transition to a Bing-based platform.

This move will pretty much see Yahoo! Search results, for both web and mobile queries, being provided by Microsoft's 'decision engine' launched last year. A 'Powered by Bing' indicator will appear at the bottom of the results page to 'mark' the use of Bing.

As agreed upon by Yahoo and Microsoft, the whole of Yahoo! Search will move to Bing technology by early 2012. The combination of search offerings will rank second after Google.
Source: Here