Someone will probably point out that data can be exported, but that is frankly irrelevant given that the URLs for your project are now embedded around the entire world to a domain name you don't control (github.com), and which your goal should be to "outlive". There is tons of historical content sitting on SourceForge and Google Code that has links all over the Internet pointed at, and it is a major issue that you can't migrate and take these links with you; if github.com offered "bring your own domain" as a feature for some moderate cost, it would be much more viable as a platform for people who care about the longevity of their projects.
