According to this post on the Zend Developer Zone website (a press release from Zend), they're classifying Windows Server 2008 as being certified "PHP ready".

This announcement is the latest in a series of collaborations between Zend and Microsoft aimed at delivering high performance and reliability for PHP applications running on the Windows Server platform. Several of these enhancements have been contributed back to the PHP community.

As a part of the project, Microsoft has even put effort into ensuring that several of the popular PHP packages (like Mambo and WordPress) work right out of teh box with a Windows Server 2008 installation.