European data from EC E-Communications Surveys.
U.S. data from Pew Internet & American Life Project Home Broadband Adoption 2007 and 2008, and U.S. Census for October 2007 data, as reported in NTIA 2008.
Korea 2007 and 2008 data from "Survey on the Computer and Internet Usage." Ministry of Information and Communication, National Internet Development Agency of Korea. February 2008.
Japan 2007 data from a presentation by IMPRESS R&D/Internet Media Research Institute.
Gartner report from data provided in its press release. Unfortunately, the release does not explain the methodology, so I do not know how much to trust the 2007 estimates. Projections in such a dynamic industry are probably not very meaningful.