What we haven't known is precisely when the mounts will be usable (though Ulduar's Halls of Stone, a level 77-79 dungeon, was said to require flying mounts), and whether the "mount mittens" (or whatever they actually implement) will be a tremendous gold sink. Today Bornakk puts as all at ease, by saying that flying mounts are currently set to be usable in Northrend at level 77, you will use the same mounts currently used in Outland, and it is not expected that "a large sum of gold" will be required (for another tier of mount skill or something). As always, this could change before the expansion goes live, but for now, it seems that we can look forward to not spending several thousand gold on griffin defrosting equipment.
will not be usable straight off the bat in Northrend. The reason for this is that they don't want us little level 70s hopping onto the frozen plains, mounting up, and zipping right off to Dalaran or Arthas or wherever; in short, they don't want us to skip content. After all, imagine if you had a flying mount at the beginning of Outland – you would've gone to Shattrath even sooner. We also know from the WWI Q&A live blog that there will be some kind of "attunement" process for getting our fliers working up there, which has jokingly been referred to as "mount mittens" (it's cold in Northrend, you know).