The Pew Internet & American Life Project released a new report today: The Ever-Shifting Internet Population: A new look at Internet access and the digital divide. Use this link to download the full-text report in PDF or to access each section of the table of contents. Amanda Lenhart, the Research Specialist who authored this report, "finds that 24% of Americans remain truly unconnected to the online world. They have never tried going online and are often quite removed from the connected population."
See also these March 2003 Internet Usage Stats.