Antitrust regulators in Washington have requested more information from Microsoft and Yahoo! regarding their recently announced search deal. Announced in July, the deal would have Bing powering search on Yahoo!'s web properties, including Yahoo!'s main search. In turn, Yahoo! would run advertising, though adCenter would still be used as the paid search platform.
Regulators are primarily concerned about two areas: competition and advertising. They're worried that the deal won't be good for competition. And they want to see that ad prices won't be artificially influenced as a result of the deal.
Of course, the argument for the deal is that combining the number 2 and 3 search engines into one provides a stronger competitor against Google, which holds upwards of 70% of the search market share. The result of a stronger 2nd place could help lower prices in the search ad market.