A post from Howard Owens reminds me that a lot of people probably don't know how easy it is to get circulation information about U.S. newspapers. For papers that are members of the Audit Bureau of Circulation, basic numbers and some deeper data is available online in a searchable format.
I find the Reader Profile information useful in illustrating some of the demographic weaknesses facing print. Reader Profiles are the result of surveys commissioned voluntarily by the newspapers, conducted by third parties such as Scarborough Research, and audited by ABC. They attempt to measure readership, not circulation, and the information is broken down by a number of demographic measures.