In the first half of 2006, only 53 percent of Clear Channel's revenue came from its radio broadcasting segment, regulatory filings show. Most of the rest came from outdoor advertising -- which could be what potential suitors are really after.

In going public, Douglas Emmett chose the opposite path of many of its fellow Reits. Several other large Reits have been taken private in the past year or so, including CarrAmerica, which was bought by the private equity firm Blackstone Group for about $5.6 billion. There are several reasons for the trend, including the perception among some investors that the value of Reits' publicly traded shares was low compared to the value of the underlying assets, which can consist of office properties, shopping centers or other kinds of real estate.