Quote from: Blizzard (Source)

We’ve also said that we are adding in more outdoor PvP objectives in Wrath of the Lich King as well as a new Battleground, a new Arena, and an entire zone (non-instanced) just built for PvP. I would like to think that those helping test all of these are going to give them thorough run throughs and submit some feedback on how to make these all the best they can be. We want to both encourage PvP and simultaneously allow for people to make choices on how and when they engage in PvP. For PvE realms, the availability and choices of when to PvP or not, is easy to figure out. On PvP realms, the chances of engaging in combat anywhere at anytime are still there. The choice to do so still comes down to players. If you feel that flying mounts cause issues when it comes to PvP, then perhaps you’re one of the ones happy about the fact that flying mounts aren’t going to be readily available for use in Northrend from the start. (At least that is the plan at this point in time.)