There is a WoW Census site out there. Though it may not be the most accurate. Since it requires someone with an addon to scan the server on a given day. And even then, it's only a snapshot of a server's population via what side you're on (I don't have the addon, but I'm assuming it checks just the side you're on via the chat channels and so forth). So a day where a lot of players in general are on might record say, the Alliance side, could be inflated while on a day with fewer players on, could record fewer Horde players. This despite the fact that on the day the Alliance side record XXXX number of players, the Horde side could have been doubled that.
I will say from personal observation, the PvE server Aeies Peak is overwhelmingly Alliance (of course, it's also a full server so no more players). Another PvE, Muradin, is also Alliance bias (though not overwhelmingly so like Aeies Peak).
The PvP servers I'm on all seem to be Horde bias. From a low pop server like Darrowmere (where TFFers first started), Gurubashi (though the server is more 50/50... well, maybe 45/55 with Horde having more), to the current (though I've all but abandoned it) TFF server, Bloodscalp.
As for Horde winning most BGs, seems to be true in a lot of servers, even those with Alliance majorities. Might lead credence that the people that play Alliance are of the younger side of things. There are exceptions though, like when they hit 70 (80 now?) where the Alliance seem to be doing better for some reason. And some BGs, for whatever reason, are better suited for the Allies (Warsong Gulch).
