Matt Asay just posted this article based on a report by SitePoint and Ektron (thanks Martin Bähr for the heads up) about how open source software is starting to become the dominant way to manage content on the Internet.
A huge swath of the web is managed by open source, with the vast majority of the remainder well-positioned to be consumed by open source.
Note: this is "managed" not "served." The web server war was already one by the Apache web server. I think these statistics say a couple of things:
Open source content management software is out there and performing
The demographics of the web are changing. Call it Web 2.0. Call it the long tail. Lots of individuals and small organizations are publishing like crazy and the tools they are choosing are open source. Small groups in large organizations are doing the same thing.