Bornakk - the latest addition to the World of Warcraft Community team - responded to a thread today regarding flying mounts and a player's request to allow their use in Northrend starting at level 70.
From a design standpoint, we don’t want people to just fly over continent and skip things when they are leveling up. Instead we want people to explore and experience the environment around them when they first reach the shores of Northrend. You will walk (or ride) from one quest to another as the zones unfold before you instead of just flying over it without batting an eye.
At some point though you will definitely be able to fly in Northrend. We are currently thinking around level 78, but haven’t made a final call on this yet.
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So it looks like right now players can expect to be able to use their flying mount starting at level 78, but this is of course still in the decision process, so it could be higher or lower once the expansion launches. Thoughts?