Ok I'm opening this back up for discussion since I've been home and trying to relax and just re-read HBP out of boredom...
I'm too lazy to quote but someone mentioned Mundungus Fletcher stealing some stuff from Sirius' house. I believe one of these things Harry saw fall out of his suitcase was a cup...and one of the horcruxes is a cup. So, it'll be interesting to see if Harry has to track this cup down that Mundungus probably has illegally sold or hidden.
I also think the theory about the locket in Sirius' house is valid as well.
I really REALLY like the idea (sorry I can't remember who said it!) that Snape has made an unbreakable vow with Dumbledore to protect Harry. Like someone said, the Order have all assumed there was a solid reason for Dumbledore to trust Snape, and this makes the most sense and really adds conflict to the story. I think it's going to come down to Snape having to choose to break the unbreakable vow for the good side or the bad side...he's going to have to choose to save Harry or save Draco, and his choice will finally reveal which side he is truly on, just as he dies (I think the book says if you break the unbreakable vow, you die, right?). Like many of you, I like the guess-work of trying to figure out what side he's on, but I was still MAJORLY pissed when he killed Dumbledore. I still don't think Dumbledore planned his own death with Snape. Maybe he faked it, but if he did I don't think she would've gone into such detail about his funeral, the phoenix lament, the portrait showing up, etc. It's like she's saying "See, Readers, he IS dead." I still stand by my theory that Dumbledore knew Snape was up to no good, but for Harry's sake (and because Snape made an unbreakabke vow with him??) he went on trusting him (or pretended to). I don't think Dumbledore can be fooled.
Anyone else re-read it and have some new or expanded theories?