It looks like flying mounts will be available in Northrend, per a post by an EU Community Manager, Vaneras. There still haven't been final decisions on when you're able to use them, though. At BlizzCon it was mentioned that you'll likely not be able to use them right away, but will be able to eventually.
Flying mounts will be available in Northrend.
However, 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.
There is a lot of work being put into quest lines, locations, events and more for the expansion. We want to make sure people can get the full benefit of all of that hard work.
If you want to see the original post, or join in on the discussion yourself, you can find the thread on the EU General Forums by clicking here.