The wikis are git repos as well. Sync them periodically and you're good. Plus it's useful as a backup, because I don't think github archives your reflog.
Issues are normally mirrored to e-mails (caveat: you don't get mail for your own comments), so you can mostly pick up existing threads if your e-mail address book can find the github users involved. If they didn't obscure recipients (at least within an organisation — because I don't think address-book lock-in is worth inconveniencing paying clients), and made an auto self-bcc of your activity, issues would be entirely disaster resistant.
If you have GitHub enterprise, it is run on your stuff and you get your own Wikis, issues, unlimited repos, etc. Even your own Gist that you can wrap behind your own CAS.
Issues are normally mirrored to e-mails (caveat: you don't get mail for your own comments), so you can mostly pick up existing threads if your e-mail address book can find the github users involved. If they didn't obscure recipients (at least within an organisation — because I don't think address-book lock-in is worth inconveniencing paying clients), and made an auto self-bcc of your activity, issues would be entirely disaster resistant.