Many had been expecting there would be an antitrust investigation of the planned purchase of DoubleClick by Google. The Federal Trade Commission has opened an antitrust investigation.
Legal experts expect the acquisition to be approved because advertisers are free to move to other advertising firms and there is a good deal of competition in the online advertising market as shown by 4 large deals for advertising firms made after the Google bid for DoubleClick.
Here are some articles with more details:
FT.com / Mergermarket - Google-DoubleClick: FTC takes on deal; client switching could address anti-trust concerns — analysis
San Jose Mercury News - FTC begins antitrust review of proposed Google-DoubleClick deal
Google Deal Said to Bring U.S. Scrutiny - New York Times