From the announcement (via email):
Nature Publishing Group introduces Scientific American’s online archive for institutional customers on nature.com at www.nature.com/scientificamerican/archive. The Scientific American online archive contains the complete collection of Scientific American from May 1948 to December 2005.
Containing over 650 issues and more than 20,000 articles, the Scientific American online archive offers readers an insight into social and scientific trends throughout this period. Online users can search through every issue of Scientific American back to May 1948.
Site license access to Scientific American’s online archive can be purchased as two collections: Scientific American archive: 1948-1992 (528 issues and approximately 15,640 articles) and Scientific American archive: 1993-2005 (156 issues and approximately 4,680 articles). Collections contain content from Scientific American, Scientific American Mind beginning with its premier issue in January 2004, plus all Special Issues. The articles are available as PDFs.