Well either Harry or Voldemort have to die, or so says the profecy, but taking in account the other books, bye drinking the unicorn blood it did say he is stuck between this world and the next, always wandering....but then again, im going to guess that Harry just out right kills him in a duel. SHALL BE A MIGHTY AND GRAND ONE AT THAT!! but yeah.
Harry won't kill Voldemort. If he does he stoops to his level.
What I think will happen Harry will transfigure Voldemort into a rock, put an unbreakable hex on the rock so Voldemort can't get out and keep it on his desk as a paperweight when he becomes the new Defense Against the Dark Arts Teacher.
Just my opinion though
Of course Voldemort has a soul! Drinking unicorn blood simply caused him to be the victim of a curse, on which no information is given. Probably Firenze meant that the restorative properties only gave a decrepit half-life. Voldemort is fully alive again no, due to the bone of his father, the blood of Harry Potter, the flesh of Wormtail...
I think people are over estimating Neville Longbottom. This isn't the Neville series this is the Harry Potter series, and it would be stupid if anyone but Harry or the combined effort of Harry and his Friends ended up killing Voldemort. I think Neville's big roll in the series is that he will become a much greater wizard than expected. He'll probably end up dating Ginny, and hopefully he gets his revenge on those death eaters who sent his parents to Saint Mungo's. Of course this is not to say that Neville cannot be the one to kill Voldy, its just a personal belief of mine that it won't be him. I guess only time will tell...