At MSNBC's "iPredict" feature, you can vote on whether you think Harry Potter will live. (No spoilers.) There realy hasn't been any significant fluctuation since the voting started in early June.
This leads me to believe that people aren't taking the spoilers that are out there seriously (although I haven't read them, and I do not wish to), because if people knew whether Harry Potter was going to live or die you'd see a trend in one direction or the other. Then again, I suspect that prediction markets which involve actual money changing hands are better at predicting things, because people have a bit more incentive to make a correct prediction. I am not going to go seeking such prediction markets because I don't want to know the answer, and I have enough faith in prediction markets that I suspect they may know!
For what it's worth, I think he'll live -- I just don't see J. K. Rowling killing off a character loved by millions (although then again, she did kill off Dumbledore...) -- but I think that at some point it'll look like Harry will die. I don't think that he'll die and then be resurrected on the third day, but it would be kind of interesting if that happened.
What might be interesting to see is the probability, at any given moment in the book, that Harry will live. I'm inspired by the graphs at fangraphs.com, which show the probability of each team winning a baseball game after each plate appearance. (I swear this isn't a baseball blog!) It's not entirely clear what this means, though. The baseball probabilities are computed by looking at a sample of how things have gone in past games; there is only one Harry Potter. Could you compare it to other books? Could someone halfway through the book think "hmm, in most of the books I've read where it's been like this, the protagonist dies, so things don't going to look good for Harry?" But of course this couldn't be made into a prediction market, because the whole book is released at a single moment.
But what if you had some sort of medium where the story is released in pieces? Prediction markets for plot details of television shows -- even if you didn't let people trade during the episode's airing (because many TV shows air at different times in different places) -- could be interesting.