I would bet quite a lot on a PG-13 rating. There's no way they would risk excluding a huge section of their target audience by including enough violence to earn it an R-rating! To me, the source material is relatively PG, although I could be primarily recalling Tolkien's narrative tone being so geared to a younger audience. However, I expect that PJ and co. are already making so many efforts to keep The Hobbit in line with the tone of LotR and re-capture the movie-only fans from the trilogy that they'll throw in a goblin beheading or two if the original cut of the film only gets a PG rating. If they're expanding the Battle of Five Armies already, though, I'd think that that alone would get them an immediate PG-13 - PJ's style isn't really of the bloodless-battle Narnia variety
I can't think what else would jack up the rating besides violence - dwarf swearing? Drunk elves? Evangeline Lily's "Tauriel" in see-through outfits a la Arwen-TTT-dream-dress?