A lot of people on the vvarious message baords seem to think it's Hary or Voldemort, even though JKR said it wasn't. Some people think Hagrid, which makes some sense. I actually think it's Filch or Firenze.

JKR said that in Chamber of Secrets, Harry makes a discovery that he doesn't fully understand until Book 6. I think the discovery was that Filch is a Squib and that is what will come into play in Half Blood Prince. Other times when seemingly background characters become important, once it happens you generally have an 'aha!' moment and it's not too shocking. You start to understand and question the background characters motives and the new surprise fits.

So let's ask some questions about Filch. Why is he at the school? He hates the students, has to do a lot of manual labor since he's a squib, doesn't seem to like Dumbledore, and is always fighting with Peeves. Why would he stay there? This is a gaping hole that may very well be filledin Book 6, and that is why I think Filch is the Half Blood Prince.

Now that I'm re-reading Order of the Phoenix though, something in the back of my mind think that Firenze might also be the half-blood prince given his brief introduction and that we don't quite understand his motives.

Ramble, ramble. What do you guys think?