Alright, let's eliminate those that it CAN'T be. First, Rowling has explcitly stated that the HBP is NOT Voldemort, therefore, niether is it Tom Riddle. Also, it's clearly a boy, hence the term "Prince". Judging from the name (Half Blood Prince) the HBP is one that has both magical parents and one or more muggle (non-magical) grandparent. (This definition of Half-Blood also comes from Rowling herself) Therefore, the HBP also cannot be Semus Finnigan, Dean Thomas (read cut information from Dean's background), Ron Weasley (or any of the Weasley's for that matter, because they are pureblood), Draco Malfoy, Gregory Goyle, or Vincent Crabbe (those three are purebloods too, as they are in Slytherin). It could possibly be Harry, but as Rowling has denied that also, we have to take her word on it and throw him out of the mix. I just thought it best to make sure that people knew who it couldn't possibly be, before they started making uneducated assumptions about the workings of Rowling's mind.
Alright, also, Rowling states that the HBP was, in fact, introduced, and played a part in COS so I need to actually reread that book before i make my official guess. Hmmm, this could take some thought, luckily thought I've got a while until it comes out to formulate my official predictions.