Yahoo! pblished an update on the progress of "transitioning ad customers from Yahoo's Panama platform to Microsoft's adCenter before the start of the holiday 2010 season". According to Yahoo officials:
"Our goal continues to be the completion of transition in the U.S. and Canada before the start of 2010 holiday season. In late summer, we intend to offer a window of several weeks during which you can choose the time to initiate and complete your transition. Of course, we'll continue to keep you informed and aware of any changes that you may need to make to prepare your Sponsored Search account for the transition.
In the meantime, there's nothing you need to do with your account, other than to continue to manage it as you've been. Keep in mind that you still need your Yahoo! account, so that you can continue to receive the traffic that we can provide between now and the transition!"