Github still holds quite a lot of nines in terms of uptime. It's just that it's extra visible when something big like Github goes down.
The important part you should consider is to switch go git. I'd recommend starting to use Github, and if you find that it's down too much, look at alternatives or at hosting a solution yourself.
When moving from svn to GitHub what you're actually doing is moving from a centralized svn system to a centralized git system.
The big difference is that in the second case you can keep working on your local repo without touching the central repo, at any time add new remotes to your local repo and pull and push from your peers.
If GitHub is down, you just keep working. If your svn server is down, you just pile your local work waiting for it to come back up, the tool will not help you in that case.
Moving from svn to git is a no brainer, even if you keep using it as if it were svn most of the time.
If Github would only host git repositories, you'd be right. But people use Github for the issue tracker, source browser, code review system. Those are just as centralized as the svn server. And in my opinion, they are at least as important as a source control server to get things done.
Apparently they have been ddos'ed multiple times recently. I wouldn't have noticed, if it didn't appear on HN though. My impression is that they have people who are quite capable of dealing with these issues. I would rather have a provider that gets under attack, but has the resources to mitigate it, than one that is rarely attacked, but would be destroyed by it.
Well if you're only using GitHub for hosting the repo then you can still work with your copy of the repository while GitHub is offline (since you're in distributed not centralise version control territory).
Git has a file protocol so you can also just sync your changes between one another via a network share of your repo. Or SSH or email each other pull requests.
The important part you should consider is to switch go git. I'd recommend starting to use Github, and if you find that it's down too much, look at alternatives or at hosting a solution yourself.