Naturally there have been quite a few complaints about this, no mount collector likes to be cheated out of a dragonhawk, nor out of an awesome jade kite, or even an albino drake. Blizzard's Community Management team have been swift in reassuring mount collectors that a hotfix is coming, that it's a known issue, and that there should be something in place by Tuesday (if not before).
So if you are getting mount achievements, but no rewards, hold on to your hats. They should hopefully mysteriously materialize in your presumably TARDIS-like mount carrying facility before long. As Vrakthis said, it should just happen when you log in, so keep an eye out and you'll be rocking a new ride before you know it.
The full blue post after the break.