I've stumbled upon three excellent discussions of Half-Blood Prince at Michael Bérubé Online, Easily Distracted, and In Medias Res. (Don't read them until you've read the book--spoilers galore!)
Wow. If only George Lucas had had one-tenth of Rowling's storytelling capabilities while writing Episodes I-III!
Incidentally, today is JKR's birthday, as well as Harry's (as evidenced by the calendar and glass of champagne on her desk). Many happy returns!