It's obvious Regulus Black, Sirius' brother, wrote the note. He used to be a deatheater and "disappeared", assumed dead, after he went against Voldemort's wishes once. I think he may turn up in the last book. J.K. has all but said RAB is Regulus Black.
My teeth hurt when I think I have to wait 2 YEARS for the final book.
I think Voldemort is truly dead, but he will have more of a role to play - there are the portraits in the headmaster's office, you know. He's got to show up there to help out a little bit.
Didn't you love all the "snogging" in HBP?? Although, I did think the ending of the book between Harry and GInny was a little too much like Spiderman (Peter Parker & Mary Jane)