My brother works at Circuit City and his store has put in it's order for (ready for this?) Harry Potter and the Half Blood Prince and all he knows is the half blood prince isn't Harry or Voldemort. My theory is the halfblood prince is Dumbledore. firstly if you rearange the letters in his name it spells "Mudbleedor" and secondly he's mentioned his brother Aberforth isn't that good at magic. So perhaps his brother is a muggle.
I've also heard a rumor that Dumbledore can only be killed if someone kills Fawkes first. this is an interesting theory I think. my own thory after reading the 5th book is that Voldemort WILL kill Dumbledore giving Harry the strength and courage to defeat Voldemort by killing him or binding him with the ancient magic that once made Voldemort unable to touch Harry because of his mother's sacrifice.
So well that's my theories and little known knowledge.