Actually, as far as I can tell, it has nothing to do with "official" GitHub. It's some guy with the GitHub username "letsmake" using GitHub pages [1].
I really wish GitHub would start doing something about people using that feature to make it look like they're part of (or speaking for) GitHub. It's incredibly misleading some times.
Sites that allow users [to have significant control over] personalised subdomains are also problematic for things like NoScript and other whitelisting tools. Since I have github.com permitted, all the subdomains inherit that permission.
I'm not even sure what a good model/UI would be to selectively white/blacklist subdomains.
I really wish GitHub would start doing something about people using that feature to make it look like they're part of (or speaking for) GitHub. It's incredibly misleading some times.
[1] http://pages.github.com/