Neth says no, of course -- she says that flying away from (or into) a fight is just another method PvPers have in their arsenal. And a few commenters in the thread make the point that I would: odds are that if someone is running away from you, it's not really a fair fight -- flying mounts may have ruined ganking, but they haven't ruined actual PvP.
Of course, what people really want here is a way to attack someone in the sky, and fortunately, with Wrath of the Lich King, your prayers will be answered -- not only have we seen flying vehicles fighting in the air, but Blizzard has confirmed anti-air capabilities. If you're just looking for a gank, and are angry when that level 64 runs away from your kitted-out 70 Rogue, you're probably still going to be unhappy. But if you're looking for the fight to go up in the air when your opponent does, Northrend probably has what you need.