Yahoo recently sent out an email to their advertisers that discusses some of the organic Yahoo to Bing transition topics. The email goes over some of the basic questions about the transition and gives some pointers, but it adds some appreciated insight into the launch of the Bing search results in the Yahoo engine and dates of when they will begin testing.
In the email, Yahoo confirmed that they are currently testing the Bing powered results "offline", but today, for the first time, Yahoo confirmed that they will be testing the Bing powered results in the live search results this month.
The email said:
"Though much of our testing is already happening offline, this month (July 2010) we'll also test the deliver of organic and paid search results provided by Microsoft on live Yahoo! traffic." Yahoo also said: "testing volumes will fluctuate during this period" and they will ensure to keep the "paid search volume in particular kept low" so that it will not impact their advertisers adversely.
It is assumed that the complete switch over will happen sometime in August or September, depending on how the testing goes, but you should keep your eyes open for the Bing search results on Yahoo as early as the next few days.