Zarhym has come out and said there are no PvE realms under populated, and that's the reason why Blizzard has introduced a new PvE realm, and not a new PvP realm. He goes on to point out that while some PvE realms are not having queues, they are only a few hundred people away from having to suffer through them every evening.
And while there are PvP realms that have queues, there are "several PvP realms that are a little too under populated."
So what does this mean?
First, it is a very clear indicator that people prefer to level their characters and immerse themselves in the game in a PvE style.
Second, it means that the game is going strong enough still to warrant new server openings; unlike Age of Conan which just combined servers. While the PR on combining servers is terrible for a company, the actual effect of isn't usually too bad. If it weren't for all the bad press combining servers would get for Blizzard, I have to wonder if they'd combine a few low population PvP servers.
Finally, I think this helps illustrate a significant reason why PvE to PvP transfers were finally opened up a few months ago. PvP servers need more population, and PvE servers seem to be what the majority of people want. If you think about it, a good way to get people to go to PvP servers is to open up transfers, thereby increasing the PvP servers populations overall and alleviating the stress on the PvE servers. Now since Blizzard is still having to opening up new PvE servers, one has to wonder if the PvP server transfer option is a success in this regards. My personal opinion is that is has been, due to providing more options for the playerbase, but I'm sure others might disagree.
Nonetheless, it's quite interesting to know that the PvE realms reign supreme in the Blizzard universe.