BlizzCon didn't allow the use of flying mounts, CM Vaneras on the EU forums tells us that we are going to be able to use our flying mounts in Northrend -- however, we may not be able to use them right away. Says Vaneras:
...it has not yet been decided if they will be available as soon as you get there or how far into your exploration you will be soaring through the icy skies. The reason for the potential delay in free-roaming is to allow the development team a chance to design and create exciting and interesting content which people will not simply fly over and ignore.
What sort of restrictions there will be on flying mounts in Northrend is yet to be decided, however. Will it be a level restriction or perhaps require some sort of quest? As soon as we find out, we'll let you know!