Microsoft has been cooking this up for over a year and working with other folks in the PHP community. I haven't run any benchmarks against it, but it does respond faster in the limited testing I did (not having a lot of PHP applications) on my XP box.
For those that are more comfortable managing IIS over Apache, this makes PHP a more viable option over ASP or ASP.NET. It's a smart move on Microsoft's part to embrace PHP over IIS as it would allow them to gain traction in the general web marketplace where Apache still is the dominant player over IIS.
Since early 2006, Microsoft and Zend have been working together on a technical collaboration with the PHP community to significantly enhance the reliability and performance of PHP on Windows Server 2003 and Windows Server 2008. As part of this collaboration, the IIS product group has been working on a new component for IIS6 and IIS7 called FastCGI Extension which will enable IIS to much more effectively host PHP applications.Download: FastCGI Extension for IIS6.0 and IIS5.1 (32/64 bit)
Today Microsoft is eager to announce availability of the Go Live release of Microsoft FastCGI Extension for IIS 5.1/6.0 (FastCGI Extension) as a free download. The Go Live release is the last step in the Microsoft beta process and represents the highest level of quality and reliability. For the first time, customers have a license that permits them to deploy the FastCGI Extension on their production Internet Information Services 6.0 (IIS 6) Web servers.
Link: FastCGI Forums on IIS.net
News source: IIS.net