Based on conversations I had today.
From the AP STYLE http://www.apstylebook.com/?do=view_recent_ask
Q. What’s the reasoning behind changing your long-held position from Web site to website, but keeping Web page and Web feed as two words? – from Chicago, IL on Fri, Jun 04, 2010
A. rpting response from May 27: the Web, shortened proper name for the World Wide Web, as in Webster’s. The dictionary also has separate listings for Web page (two words), and website (one word).
From Ask Oxford
http://www.askoxford.com/asktheexperts/faq/usage/website
From Wikipedia
http://en.wikipedia.org/wiki/Website
Used on the W3C website