In EQ2 you could Defect from one side to the other, Freeport to Qeynos, and vice versa. It took completion of a fairly long questline which started with defecting from your City and becoming Neutral (meaning most Cities were not open to you) and included raising your faction standing with the other side before they let you into their cities.
If you could do that in WoW, would you? and which character would you do so on? or if you could, would you defect and then remain neutral, knowing that only neutral cities would be open to you?
I would send a Worgen Character over to the Horde side.