Looks like it works with `.org` documents as well! I've been authoring my README files in org-mode for a while now (richer formatting options than Markdown) and GitHub does a great job rendering the org-mode.
I'll have to test AsciiDoc (my markup of choice) when I get a chance. Now, if only GitHub mobile would start rendering my AsciiDoc files, I could use it as my mobile knowledgebase/notes viewer instead of the website version.