It works on the page and it works when embedding on a page (from a Gist), like this:
According to this: http://www.infoq.com/news/2008/03/github-git-repository-hosting
they are using Python Pygments
Chris Wanstrath shared some
information about the inner workings
of GitHub with InfoQ:
GitHub is mostly implemented in Rails.
The post-commit integration mini-apps
we’re working on are all written in
Merb, we use the Python Pygments for
syntax highlighting, and we use Ara T.
Howard’s Bj plus some Ruby scripts for
our queueing system. And, of course,
we use the Ruby Grit library to
interface with Git.
I believe they use their albino gem. It is a Ruby wrapper for the pygments syntax highlighter.