I've been using PHP on IIS6 and IIS7 for years now.
Infact, i have been running PHP and MySQL on Windows longer than on Linux. It is very stable and easy to manage in a familiar Windows environment using the IIS manager.
It was a little trickier to setup before IIS7 with the odd strange problem, but no more so than running on linux. With the latest fastcgi integration under IIS7 PHP is very easy to configure for the entire webserver and even quicker than before on Windows.
If cost wasn't an issue i certainly wouldn't hesitate using Windows over Linux for all my production environments, and im not too sure why the general PHP community has such an issue with Microsoft and Windows.
Most of us develop on Windows anyway and simply host in a linux environment. AFAIK the more Microsoft, and other vendors, embrace PHP the more production uses it has as a language.
It's a more serious attempt to provide PHP compatability to Windows than anything Mono does for .NET
I am very glad to see Microsoft offer some kind of official direction for PHP after previous steps in the right direction with IIS and wholly embrace their attempts to help PHP developers use their products.