JK has already said on jkrowling.com that the half blood prince is not Harry or Voldy. it has to be someone new 'cause think about it, all the existing characters are full bloods or muggle borns.
All Weasly's, Sirius,Lupin, Mr and Draco Malfoy, Crabbe, goyle, Neville, Luna, McGonagall, Snape, Dumbledore are all full blood.
Hermione is a muggle born.
Filch is a Squibb.
so unless it's someone in "DA" or unless it's some member of the order of the Phoenix, it leads you to believe that it's someone new. The only one whom i think it's an outside possibility is Hagrid. he was half Giant, half wizard,(which makes him half blood) so maybe he's a prince of a group of Giants. but I don't think so.