WhiteHat Security released their 5th website security statistics report yesterday. They also held a webinar to go over the results, and the website security statistics slides are also available on slideshare.

Total Websites: 687
Identified vulnerabilities: 11,234
Unresolved vulnerabilities: 3,541 (66% resolved) 
Websites HAVING HAD at least one serious issue: 82%
Websites CURRENTLY WITH at least one serious issue: 61%
Average vulnerabilities per website: 5