Prepare for battlegrounds full of these guys within three months of the release of the expansion, especially after people get to see how much ass they kick in PTR. It may be wise for Blizzard to prevent Hero Class characters from entering PVP battlegrounds and raid groups until more of them are available, so as not to create an Alpha Class.
You mean Warlocks?
Yes, I'm sure there will be tons of them for awhile, but like Jylan said, they won't be that special that they're going to obliterate everyone else. If they did that, they would just render more classes useless in PvE and PvP and that doesn't seem to be their goal.
As for the battlegrounds being full of them, a blue actually commented on this with "it'll be like that on both sides".