Microsoft and Yahoo! have received regulatory clearance to form the Yahoo! and Microsoft Search Alliance in the United States and European Union.

“Search ad inventory from Yahoo!, Microsoft, and their respective partners will be combined into a new unified search marketplace, giving advertisers of all sizes access to a combined audience of nearly 577 million searchers worldwide.”

According to Microsoft, the combined search may be implemented before the 2010 holiday season.

Once completed, Microsoft adCenter will be the platform for all search campaigns in both Yahoo and Bing.