SilverStripe CMS is the first open source web application to achieve "Certified for Windows Server 2008 R2" status.

Silverstripe is an open source project that offers the Sapphire PHP5 framework for web developers as well as its CMS for website editors which operates under the BSD license and uses several open source and open standards components - PHP, HTML, CSS, and JavaScript.

Commenting on gaining Microsoft Certification, Silvrstripe's  co-founder, Sigurd Magnusson points out on the Silverstripe.org blog:

This is notable because Microsoft has certified a very small number of web applications, and we are the first ever open source web-application to attain certification.

According to Andrew Gordon, looks after strategy for the Microsoft Platform in New Zealand, where he is focused on working closely with the open source communities:

This is also a highpoint for Microsoft Windows in our ongoing efforts to provide a truly open platform that provides the broadest set of choices for everyone. This reflects the broader landscape of Open Source and Microsoft being implemented together in enterprise customer environments, and we hope this will encourage other high quality Open Source products to work towards Windows Server 2008 R2 Certification so as to underline their enterprise credibility.

The availability of open source web tools that run under Windows is undoubtedly  a good thing. SilverStripe CMS was one of the selected products bundled with Microsoft Web Platform Installer (WebPI) when it launched last year. SilverStripe has worked to ensure that SilverStripe CMS runs fast on Windows, through being compatible with the Microsoft WinCache PHP bytecode cache, a free extension to IIS and its certification is a result of this work.

However it is interesting to note that Microsoft doesn't have a CMS product of its own and hence validating an open source product doesn't hurt it too much. It does bring PHP technology into a place that you might well expect to see ASP .NET.