who is the half-blood prince?

This Saturday promises to be an exciting day; the latest Harry Potter installment - Harry Potter and The Half-Blood Prince - will be simulataneously released at around the same time all over the world.

I must admit that I sort of forgotten how the previous book ends (might need to re-read it later...) but I'm really looking forward to know how the story will progress; we know that one of the characters will die (sniff.... so sad..) and that this particular book will supposedly answer many questions we have so far and will no doubt create new ones.

Here's a picture of the jacket illustration for the US version of the book (look at the details; tantalizing clues abound):

Ah well, I am looking forward to a great weekend snuggling up with this great book...