Okay: for all of you who have finished Harry Potter and the Half-Blood Prince (and if not, what are you waiting for?), here is a site that fleshes out conclusions that I had come to when I read the end of the book. As the URL itself is something of a spoiler, I won't list it here, but only click here if you don't want the end of the book ruined!
(Thanks to blog reader Michael Koh in Singapore for the link!)