It seems that some surnames are too rude for Microsoft. If you're unlucky enough to have a “rude word” within your actual name, you can't apply for a .NET Passport. I think they need to work on their regular expressions a little more.

I would have tested this for myself, but I can't access the Passport registration website. Apparently, the latest version of Mozilla isn't a recent enough browser to view the site. All I can see is a page that tells me I need to use a “current Web browser, such as Microsoft Internet Explorer version 4.0 or later, or Netscape Navigator version 4.08 or later.” Interestingly, that page doesn't even come close to validating against any HTML standard.