This is very often asked in General Chat in Wrath of the Lich King closed beta. Now at level 77, a couple days ago I was one of those asking with curiousity how or where it starts at. So now I pass the information onto Blizzplanet visitors for future reference.
I have added the quest chain walkthrough to the Wrath of the Lich King menu under Story Arcs. To unlock the Northrend Flying Mount you must complete a series of quests that aren’t so hard to accomplish, and you also unlock a hidden flightpath point to boot.