So... if they introduce a regression that breaks GitHub they can't repair GitHub anymore? :)
When you eat your own dog food and some poison gets in, you die and can't fix it.
Our deployment process operates outside of our traditional network setup. If, for example, we push a change that breaks repository serving for everyone on github.com, we still deploy as normal and won't be impacted by it.
For something that could break GitHub, I'd hazard a guess that they'd deploy that as a branch without merging or to a subset of servers first. In addition, their deploy tools are built off Capistrano, not off GitHub (except for the Git bits, of course).
