I read an interesting article today breaking down the top 1,000,000 web sites using WordPress. Directly from the article, here is the methodology:

To determine themes in use by the worlds most popular WordPress based websites a search of the source html from the primary page was analysed for “wp-content/themes/” this is a good indication of a WordPress installation and also reveals the theme in use. As you would expect from a Free open source content management system; of the 160438 sites we found with WordPress themes many are running free themes, but many are also running premium commercial themes.
HackerTarget.com

Desk Mess Mirrored (if you download and look at the full listing) sits tied at #347 for overall usage on the top one million sites … cool!