Yahoo announced that Bing will be powering Yahoo Search on the organic side later this week. Right now and for the past month, Yahoo has been testing Bing powered search results on Yahoo. Yahoo said, the full transition is happening later this week and they will let us know when it is complete.
You will also know the transition is complete when you see the "Powered by Bing" logo on the bottom of the Yahoo Search results.
This is only for the United States and Canada Yahoo Search results, the other results won't change over until 2012. This is also only for the organic side of things.
Also, you should know the organic results between Yahoo and Bing, the ranking order, will be exactly the same when this transition is complete. Yahoo may add some makeup to the results, but the ranking order will be exactly the same.
On the paid side, Yahoo is testing the account migration tool now and hopes to release it to everyone in the upcoming weeks.
I'll follow up this post on the webmaster side of the things. Specifically, Yahoo Site Explorer, BOSS and SearchMonkey.
Forum discussion at WebmasterWorld.