Since this vulnerability has been known about for years and experienced crackers have presumably checked for it before, is there any reason to be concerned about more malicious corruption of other repositories on github?
This attack was a combination of Rails and git/ssh keys. There's a bit of a clever aspect to it, one that isn't implied merely by understanding the vulnerabilities of update_attributes. While I think it is likely someone else has thought of it before, it is a little more exotic than something your average cracker is going to try.
The only reason why a cracker hasn't tried this is because it seems too simple to work.