Yahoo! posted the reply to questions raised after publishing update on Yahoo! and Microsoft Search Alliance. Here're some important questions:
"Is there going to be transparency between the search engines so that advertisers can see where their ads are being delivered?"
Answer: Once the transition is complete, adCenter will provide an aggregate view of Yahoo! and Microsoft traffic in your reports. Also note, however, that after Yahoo! Sponsored Search advertisers are transitioned to Microsoft's search platforms, advertisers willn't be permitted to select traffic solely from Bing or solely from Yahoo!"