Obviously, the cursed opal necklace wasn't meant to be delivered by Katie Bell... it was meant to be intercepted and handed to a teacher to be brought onto school grounds past Filch and his sensors. I presume Snape is responsible, since he's the one it ended up being delivered to. I still don't think he's loyal to Voldemort, though. Despite Dumbledore having been a moron for five books with respect to keeping secrets from Harry, I don't think he'd trust someone who wasn't trustworthy. Barty Crouch in Alastor Moody's guise notwithstanding, I suppose... but I'm giving him and Snape the benefit of the doubt here. I still think Snape will probably die in this book, though, as much as I hate speculating about stuff I technically don't know will happen. I also think even more that Voldemort is the Half-Blood Prince, which is what I've been suspecting since people started suspecting stuff (well, actually, since I reread the fourth book and was reminded that Voldemort was a half-blood). Didn't it say somewhere, way back in book 2 or so, that Tom Riddle's best subject was Potions?
Anyway, having turned my computer back on this late at night to record that probably mostly correct but ultimately meaningless conclusion, I'll go to sleep and read more tomorrow. Enjoy the book, those of you who are reading it or will in the future.