It'll be Voldemort and Harry. Harry's a horcrux. That's why Rowling said that, if she spoke more plainly about her faith, everyone would be able to guess the ending. Harry must die to kill Voldemort.
(Of course I don't know any more than anyone else; but we FReepers always speak as if we had The Ultimate Insight!)
I've always though that to be a reference to Revelation, e.g., beginning at Revelation 12.
If you start there, you'll find a lot of very interesting parallels to the HP stories (always taking into account, of course, the JKR is writing a novel).
A few others have mentioned the possibility of Harry, or even just Harry's scar, being a horcrux, but I'm not sold. When could LV have accomplished this? I'm working on the assumption that creating a horcrux is a very difficult task, and that the object to be enchanted must be present when the spell is cast. (Could I sound like a bigger dork?) Additionally, the spell has to be cast around the time that a murder is committed, right? Well, maybe not, but that's the impression I get. I just don't see when LV could have accomplished it, and it can't be done by another wizard on his behalf. Drat! I hate waiting to be right, haha