OK, I've finally read through all these posts and I get to put in my ideas!!
OK, first of all-- RAB! As I was lying in bed the night I finished the book (at 12:20 in the morning. . .) I came up with Regulus. OK, pro's and con's to this possibility:
Pro's: Regulus was a death eater, and the person who got to the horcrux was a death eater. How do I know this? Because RAB addressed the letter, "To the Dark Lord," and it has already been made clear to us in all prior books that only death eaters refer to him as "the Dark Lord." Secondly, Rowling did after all introduce the guy, and when has Rowling ever made such a big deal over a pointless character? Not that she necessarily made a HUGE deal over him, but you didn't hear her going into such great-depths about Sirius's mom. . . beyond that, if a person were to do it single-handedly, they would have to know something that Dumbledore didn't know, another thing backing up the death eater theory. All this, and Regulus is the only afore mentioned character I can think of whose initials coincide, and I don't think Rowling would introduce a completely new important character in the last book. Hence, Regulus.
Con's: Regulus was killed fifteen years ago. So, how long ago exactly would Voldemort have had to created the horcrux? It's never too clear to us when he created all of them, but some things make it sound as though perhaps Voldemort had not completed the last of his horcruxes until, possibly, recently. Such as Nagini, for instance. Also, the other thing I thought of that would make Regulus seem a slightly less likely possibility is the fact that so many of us thought of him right away. Since when have we known Rowling to be so obvious? It would seem like relatively poor planning and writing to make it so easy to figure it out (easy enough I could figure it out half-asleep.)
I'm pretty sure I'll come up with some other stuff in the future, I keep coming up with new things and forgetting others just as fast. Anywho, moving on.
OK, everybody keeps talking about how annoying Ron was, "snogging" Lavender like that. But I think everybody has it all wrong about his motive. Yes, I think part of it probably was as revenge against Hermione for her relationship with Viktor, but I think the major reason would probably actually be to prove something to Harry and Ginny. OK, think about it-- if your LITTLE SISTER gave you that harsh of a tongue-lashing in front of your BEST FRIEND, saying you had "about the experience of a twelve-year old"-- well, I think if it had been me I'd feel so embarrassed I'd be ready to go out and "snog" anybody to prove her wrong, no matter how ridiculous that term is. And Lavender was the easy target, she'd already expressed interest in him and so she fell prey to his using her to boost his self-esteem in regards to Harry and Ginny's opinion of him. Which might be why he reacted to Hermione's asking Cormac out so adversely: he might not have considered how it would affect her, being preoccupied with Harry and Ginny like that. So, no matter how annoying it was, I think it might kind of make some sense.
I'm not sure what to think of Snape right now-- I'm trying to figure out if I should hate him, or hold out hope that he might redeem himself. I really think Osaka is right, I think he might have a thing for Narcissa, which might be why he's always favored Draco so much. I mean, there have to be some other promising Slytherines, why Draco? So the only other thing I can think of concerning this issue is that possibly Snape's loyalty does not lie in the dark side with Voldemort, but maybe with Narcissa. It's possible he was willing to forgoe his allegiance to Dumbledore for Narcissa's sake. Can't really decide what I think for sure though. I think the idea about Snape having made arrangements with Dumbledore beforehand is also very plaussible.
I thought it was really interesting learning so much about Voldemort's past, it was cool learning how he got to the point he's at. And I'm actually half glad that the next book is not going to be set at Hogwarts, it will make way for some fresh new material for the finale. And I'm sorry, but does anybody ACTUALLY believe that Ginny is just going to sit on her butt at home while the Harry/Hermione/Ron trio go off and find adventure? Puh-lease! She's totally going to go with them. And lastly, I have an inkling that Draco is going to play a HUGE roll in the next book-- I think he might actually kill Snape. That would be something, huh?
Anyways, I can't wait to find out for sure, (but then again, I'm sad that before long it will all be over. . .
. . .)