The International Monetary Fund says the mortgage mess is "the biggest financial crisis in the United States since the Great Depression," which means one thing for John Wren, CEO of ad-holding giant Omnicom: acquisition prices for companies in the advertising industry are low, and it's time to get shopping. Wren told analysts on earnings call yesteday that Omnicom will get more active with is wallet “perhaps now, as the economy worsens." Last December, Wren promised Omnicom would, like rivals Publicis Groupe and WPP Group already had, go on something of an acquisition spree in the interactive space. Didn't happen, Wren said, because Publicis and WPP "aggressively paid, in our opinion, uneconomic prices." Unfortunately for Wren, that means that the only companies left are the ones not worth overpaying for.