Ok, dumb question time: Outside of things like banks, does anyone actually run a 100% SSL web server?
I thought the point of Firesheep was more "don't use unsecured networks" than "don't use websites that aren't 10% SSL." If it's the former, then this only does any good if the "victims" are the people providing the service. How many people do you think are going to notice this tweet immediately, realize where they were when it happened, and complain to the wifi provider (whose response, of course, will be "use at your own risk").
It might do some good for people running insecure networks at home, but the people that understand what happened and how to fix it would already be running secure networks at home.