Date: 09/25/07 (Java Web)    Keywords: google

A simple google search reveals over 55.5 million pages are generated using WordPress today. The WordPress search volume shows a healthy increase over time as can be seen from the graph. As an aside you will also find over 47, 000 pages displaying the dreaded error page of WordPress. However that is an insignificant volume (.085%) [...]

Source: http://blog.taragana.com/index.php/archive/stats-over-555-million-pages-powered-by-wordpress/