Google's rampant spending has piqued the interest of many, and their most recent acquisition of DoubleClick has gotten the Federal Trade Commission involved. While both Microsoft and AT&T played heavily into this, both citing “concerns”, a deal of this size with these types of companies is going to come under some scrutiny regardless. The FTC has launched an antitrust probe into Google as a result. The big issue at hand, it seems, are concerns over privacy. With the immense amount of data that Google and DoubleClick both collect, the fear is that combined the information would be prone to abuse:

The deal, involving powerful forces in their respective niches of the online advertising business, prompted privacy advocates and competitors to raise concerns after it was announced last month. Those concerns and the deal’s size made a preliminary investigation all but certain, according to antitrust experts.
Google is of course not concerned, claiming that the purchase has nothing but good intentions. Google doesn't just face heat in the U.S., about also in the European Union, where an advisory panel has been questioning their privacy practices.