From a new post on PHP Magaizne:

In a quick pointer from Jason Anglin is a post on HTML FIX IT.com that pops the question, "Does MSN use PHP and MySQL?". "...while looking at msfn.org to put in my vote for Firefox on their poll, I noticed that the site was based on PHP, an Open Source language. Microsoft usually puts down anything with a GPL license at what seems to be every opportunity, so it seemed odd that a site for Microsoft developers runs on PHP, even if it isn't an official Microsoft site. Then today Cnet put out an article speculating that MSN might be running on PHP and MySQL as well. The screenshot they show as possible evidence, shows the Brazil MSN with a PHP/MySQL error message displayed. I don't actually think the image is anything but a fraud, because Netcraft shows that server as running Microsoft Windows Server 2003 IIS/6.0 and yet the file paths in the error messages seem to be Unix paths not Windows. Still, there has been long speculation that for a long time some Microsoft servers (like Hotmail) were running BSD and not Windows on the back end, so who really knows?". Read the original story on CNet News.

While the screenshot could be evidence of some Open Source involvement behind the scenes, I'm not really sure how much stock to put in that. I know it would be the smart move for them to make a transition over to things like PHP and MySQL, but what it would really boil down to is "is the move worth it?" (I'd say yes, but that's just me...)