Yes, the problem was known before-hand by security researchers and a number of other people involved in web development. That doesn't mean it was known to users or to the developers of GitHub. In fact, given the fast response by GitHub and the tweet by pjhyett (http://twitter.com/#!/pjhyett/status/28924943340), it definitely appears that the GitHub team were not aware of this issue prior to the release of Firesheep. In any case, the release of Firesheep dramatically increased the likelyhood of this particular hole being exploited in the wild. The responsible action would have been for the author of Firesheep to notify the targeted sites about this hole with a reasonable delay before releasing the tool. His point would have still been made, but the sites that were willing to fix this would have been able to put a fix in place before users were unnecessarily left exposed.