The question, however, is what they'll be. We've also heard that Blizzard won't be allowing flying mounts at least in the early parts of Northrend-- they claim that flying allows players to skip content, and they don't want anyone skipping content. But we will be able to use them eventually, and considering the steps that Blizzard has taken (in changing the riding/mount prices), it is almost assured that we'll see a few different mount options come out of Northwind. We've already seen dragons there, and the Blue Dragonflight will be there as well, so that points toward another type of dragon (other than Netherdrakes).
Of course, there's another type of vehicle we'll be riding around on in Northrend-- those siege weapons. It may be that, as Bornakk says, we don't get another riding skill rank to train, but there may be training purchases involved in letting players drive and use siege weapons as well.