To answer my own question:

    http://stackoverflow.com/questions/8693319/where-can-i-install-gems-from-when-rubygems-org-is-down

Of course if you have private patches of gems with bundler's `:git` option, you'd need to repoint all of those, too (as well as keep them in two places).

It'd be awesome if there was some simpler way, maybe a separate declaration in your Gemfile so that any gems added to your project also get installed on a corporate git server, and then bundler uses that as a fallback if `source` doesn't work.