I noticed this at the MoP release event, and I see it more and more in game. I have been playing wow for 9 years and I find it absolutely hilarious when horde and alliance talk to each other.
This is normally what happens :
Alliance sees Horde. Alliance /waves or ignores and continues with daily life. End of story.
Horde sees Alliance. Horde /farts then chases the Alliance down and beats the living shit out of them. Brings his friends, teabags, /spits, /farts, res camps until the Alliance logs off or brings friends, kills friends, laughs and then moves on with normal life. End of story.
Now I think this is funny too because at the MoP release even the Alliance players are always friendly to the Horde, "hey you are horde?! Cool!" Horde players : "You play Alliance?! What are you 12? Get the hell away from me. FOR THE HORDE!!!!!!!111!!!!!"
How do you guys react when you see the opposing faction? I am curious.
btw I play horde, on Mal'Ganis, where Alliance doesn't exist. When I cross realm though....heh I hunt those little bastards down.