Contrary to what we originally thought there will be no flying mounts in Northrend when you get there. Blizzard has hinted to the fact that they will release the nets at 78. They claim they don’t want people to just skip over content. Personally I think the people that want to just explore are going to do it regardless and by not allowing them to fly around they made things a bit more difficult.
While Blizzard couldn’t officially comment on it and now that the game is released you can’t get any “hints” to the game, you can see for yourself that you can’t fly in Northrend. While the mount tool-tip was misleading pre-WotLK please do take note that as of now you can not use your flying mount while in Northrend.
We will also be updating our leveling guide now that WotLK is released. Our intent is to get 70-80 out as fast as we can and than update the 1-80 guide. You can get our guide right now at LiTi4.com.