Out of curiosity, is bitbucket blocked? If not, why not use them as a mirror and interact that way until github is available?
(I thought the whole point of DVCS was that your repo wasn't centralized)
GitHub has become more than a DVCS: Loads of relevant Open Source projects are hosted there. If I want to contribute - or even browse issues - I need access to GitHub. Same goes for discovery.
Also, lot's of tutorials will provide their example project on GitHub. Plus, GitHub both hosts a fair amount of static pages of Open Source projects and individuals, plus some blogs/tutorials embed gists when the use code samples.
If your release and branching is based on Git and all the work you did resides there, it becomes difficult to switch horses mid stream. If your open source contributors and have accept pull requests then the history gets lost.
Repo isn't centralized, its distributed amongst Github not across systems (Github to Bitbucket or Cloudforge etc.)
I understand if there are post-commit hooks that depend on github integration and bitbucket integration is not available.
In fact, you could add a bitbucket repo in a matter of seconds and use it alongside github. Or in place of.
Pull requests are merges. The history lives in the repo.