Who is R.A.B.? 

regulas black would be the obvious answer but beyond that it could be any random new character.

Quite nice systematic analysis.

Ok, here's my theory. Let me preface it with a few comments. 1st, I am a fan of the books and movies but would not win any trivia contests, so if I get some information wrong. Secondly, I fully acknowledge that this is a very weak theory at best. Third, I haven't had any sleep in the last 24 hours, so I was a little groggy when I came up with this. All of this being said, I submit that R.A.B. is Rita Skeeter. I'll continue when you've stopped laughing. So, I'm thinking that in the note, the writer state's, "t was I who discovered your secret." Well, isn't that what she was always about, discovering someone's secret. I know the initials don't match, but that could be easily explained away by either a maiden name, or a secret marriage to someone with the last name that starts with "B" Regulas Black, for example, the person everyone is always assuming is R.A.B. All of the theorists are stating that the writer of the note must be dead, but we don't know that for a fact. J.K. Rowling has herself stated that in the final book no new major characters will be introduced. Well, Regulas Black has only been mentioned a couple of times and in passing. His character has not really been fully introduced in past books. She has also stated that Rita Skeeter will be in the final book. So, there it is. A very weak theory as I said, but just as plausible as any other out there. Tell me what you think.

bubbaspace: I've heard weirder theories