Blizzard has stated that they'll be disallowing the use of flying mounts in Northrend for the first few levels in order for players to appreciate the geography and not miss anything in the latest expansion. Given the amount of work they've put in so far, and looking through the fantastic scenery, it seems like a fair charge. It's recently come to light that the 'attunement' process, which was under much speculation, will not be an attunement at all or cost any money.
In fact, the ability or key to fly in Northrend, called Cold Weather Flying, is a reward from a quest chain obtained in Sholazar Basin at Level 77. The so-called attunement process for flying will actually even reward money, rather than cost anything. MMO Champion has screenshots of, and Wowhead has information on the ability and quest chain. It seems like the mad rush won't necessarily be to Level 80, but a first stop at the all-important Level 77.