OK, but that doesn't really answer my question, which is why people aren't more concerned about earlier more surreptitious corruptions of the repositories of github via the same vulnerability. It's got to have been an attractive target for the likes of the Operation Aurora [1] folks.
Presumably, Github could check for this activity by finding all public key submissions in which a public key registration involved a user id that is not the same user id as the signed-in user who submitted that. I'm not sure that's a simple DB query though...
[1] http://en.wikipedia.org/wiki/Operation_Aurora