While some web stats show Mac OS at 8%, SEC filings suggest 3% of existing PCs are running MacOS. This can be deduced from the http://SEC.gov filings which show units shipped by Apple as 2.289 million in the quarter ending 2008-03-29. see http://sec.gov/Archives/edgar/data/320193/000119312508097759/d10q.htm

Quarterly sales of all PCs are 69.5 million.
see http://www.itfacts.biz/global-pc-shipments-grew-146-to-695-mln-units-in-q1-2008/10479

This shows that the web stats are biased towards hot Mac markets: USA/Europe and under-report the global picture. This affects the miserably low numbers for GNU/Linux as well. If Mac OS is 3% and M$ is 90%, 7% of PCs could be running GNU/Linux on the web.