I think this is a question of perspective: Having the master repository on GitHub doesn't mean you are in immediate danger or lose your "family jewels". IMHO it's quite the contrary: I'm e.g. sure that in case that something goes wrong with GitHub, there is far more manpower behind it to fix that than for any self-hosted repository. And you can of course have some mirror of your GitHub repo in case of e.g. an earthquake/meteor/... in the San Francisco area... ;-)