We've got an update today related to the use of flying mounts in Wrath of the Lich King's Northrend, and while none of it is set in stone, the general ideas behind the comments made by Blizzard CM Bornakk are interesting.

First off, Bornakk's comments stated that the main plan behind putting flying back into the game was to make it available at around level 77:

The current plan is to allow players to use their flying mounts once a character hits level 77 in Northrend. I understand you feel they don't add to aspects of the game that you are looking for, but we believe they do add to the game and we are building the new content with the ability to use them in mind.

Furthermore, it seems as if the flying content will be accessible either through some sort of attunement or payment, though the indication somewhat points to the former (or a possible mix of the two):

You will be able to use the same mounts that you currently use in Outland. There will be some sort of process to complete to be able to use the mount once a character hits level 77, but the exact details are undecided at this time. If you are referring to needing a large sum of gold to use them, sort of like a new tier of flying skill, that isn't the current plan.

All in all, the ability to get back to flying at 77 sounds good, especially if you've already got your epic flyer. Keep checking QJ for more updates on this as it becomes available.