That's about 1 site for every 3 people in the United States.
September 2006 Web Server Survey
"Growth is being driven by two trends: the popularity of blogging services, and the heated battle between Microsoft and Google for new users for their web platforms. Huge growth continues at Windows Live Spaces, Microsoft's free blogging/networking service,"
I'll bet you a 0-day that most of these are splogs.
How do you go about securing millions of websites? There is no silver-bullet solution to look forward to. We'll have to use everything in the arsenal. (security in the SDLC, security assessments, web application firewalls, source code audits, secure configuration, catchas, browser toolbars, etc. )