I actually expected Horde to be popular, especially on PvP servers. In SWG the rebels had the majority on every server, EXCEPT the Euro servers (Chimaera at least, not sure about Farstar) where the imperials had the majority. And the WoW posts/polls on prydwen.net seemed to indicate that a lot of people wanted to play Horde.
Our server might be the exception, but I guess Horde has the advantage on most PvP servers, while Alliance has the advantage on PvE servers. I'd have to look at more stats to see if there's any thruth, but it's what I expect.