I just found the
WoW Realm Census Page. I was surprised to see that on Zul'Jin (our server), the alliance outnumbers Horde nearly two to one. I had heard that Zul'Jin was one of the most balanced servers, and with my alliance alt I've even heard them whining about how they are always outnumbered by the Horde and always lose in Alterac Valley. I'll let you draw your own conclusions from that.
It turns out that Zul'Jin IS one of the most balanced ... of the PVE servers. (Most PVE servers seem to be over 70% alliance). Same goes for the RP servers, but on the PVP servers, it is much closer to 50-50, and the Horde even outnumbers the Alliance on a good number of PVP servers (including a glorious 64% on the Gurubashi server). This seems to confirm the theory that the hardcore gamers are more likely to play Horde, and the quieter role-players (I am trying to avoid saying limp-wristed weenies) break toward Alliance.