Since Bethesda is doing it, you can pretty much guarantee that it's going to be first person. But.....it doesn't 'have' to be horrible. I just whipped up a picture to show how they 'could' do first person without destroying turn-based, targeted combat:
http://www.angelfire.com/mn3/josefgagnier/images/F3example.JPG
This is all considering it's turn-based (man, I hope it is).
For a targeted shot like the picture shows, you point your "crosshairs" (really just a cursor though) to the point on the body you want to shoot. If it was a non-targeted shot, you'd just click anywhere on the guy and it would calculate how Fallout already does it.
For the moving around aspect of it, it could kind of be like the Wizardry VIII way of moving, except that you don't lose AP points if you decide to backtrack in one turn. The farther you move from where you are when the you started the turn, the more AP you use up (like in the originals, but instead of a number next to a hex indicating how much it will take, you can just see the AP go up and down in "real-time" when you're moving around, figuring out where to stay put for that turn). I would assume that an invisible grid of some sort would be there to determine how many AP's you use up.
Since you're never directly controlling a party in Fallout, your NPC allies could just determine range, movement, weapons, and AP's, and move on their own. I suppose you could bark orders too, but that's their call.
Let me state that I really, really wish and hope that Bethesda does not stray from the successful model that we fans have come to enjoy. But since they probably are, let's just hope for the best.
Also, that picture sucks, I know, but I just thought I'd throw it out there.