Usually, PHP development is occuring on Windows, but deloying to Unix, or Linux servers. It's because, performance issues of PHP on Windows.

Personally, I have tested PHP on Windows/IIS environment with FastCGI interface.
Basically, all of functions worked well. However, response time was slow against Linux/Apache.
So, I hoped to get improved the PHP performance on Windows for several months.

At the moment, we have a good news.
http://www.infoworld.com/article/06/10/31/HNzenphp_1.html?source=NLC-TB2006-10-31

You can leverage PHP on Windows. I think it's perfect combination for PHP developers. You can utilize the benefits of Windows, ease of use/manage, performance, and security.
Windows vista is around the corner, so it means it's not too far the birth of Longhorn Server.

It's really great time to use Windows.