Who is the the Half Blood prince?

[Ginny Weasly]

Em's right. And J.K. can't do whatever she wants, she has rules to follow, you know.

[Fawkes]

I think the Half-Blood Prince must be half-blood..  None of the previous titles were useless.. I mean the Philosphers Stone was all about the Philosphers Stone, the Chamber of Secrets was all about the Chamber of Secrets.. etc. I think you got my point..

Anyway so what we know is that the person is a male (prince) and he's half-blood.. I really think it's Neville .. I have a feeling he's going to be very important in the next book. :)

[Tweetie]

Well since we are doing wild people I think its Justin Finch

It can't be Jusin because in the second book he said that he was Muggleborn.  The prince is obviously a half-blood, (hence the title), so Justin is out.  Seamus seems like a reasonable possibility.  The first book said that he was defintitly a half-blood.  :)