GameSpy has posted an interview with J. Allen Brack, Lead Producer for WoW, in which Brack talks a bit about flying mounts in Northrend:

We do have a plan for flying mounts, and letting players unlock their flying mounts at level 77 is going to be acceptable to the players once they get there. There’ll be either a quest or a trainer that you can visit to unlock your flying mount which we’ll expect you to use for your last three levels.
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You can use your Outland flying mount, and we will have new flying mounts for you to get as well.

Brack is pretty clear, however, that we will not be using those flying mounts in the older parts of Azeroth.

The old world is still grounded. The reason for that is just that was how the world was made. The world was not made with the idea that you could fly around. There’re huge amounts of trickery used to design the world. There’s lots of empty stuff that just doesn’t make sense and wouldn’t look good if you could fly around.