Now I know this topic has been commented on more times than the 'wardrobe malfunction', but I figured I'd add my two-cents anyway.
Now, thanks to Jo, we can rule out the two people who our minds immediately jumped to when we learned the title: Voldemort(or an form thereof) and Harry himself. Had the title applied to Harry, it is just further proof to the theory that he is the heir to Gryffindor.
So, let's think of some of the half-bloods we have in the Harry Potter universe. The first name that comes to my mind is Hagrid. His father was a wizard, and his mother a very prolific giantess, making him a half-blood.Could his mother have been 'royalty' among giants? And if so, could this make Hagrid the Half-Blood Prince?
Dean Thomas is next in line. He hasn't been the most developed or mentioned character in the series, but you are always aware that he is there. Could Jo just be biding her time and then WHAM! hit us with a huge revalation about him? The same applies to Seamas Finnigan.
Me? I think that the HBP is a character we have yet to meet. Could they be an heir to Hufflepuff, Ravenclaw, or Gryfindor? I do not know. It makes sense that, being sooooooo close to the end of the series that the HBP be either a new character, or one we already know well, but who has"skeletons in the closet' shall we say?
Well, that's it for my first collum! I'll be back though!