We're uncontrollably worked up over Harry Potter and the Deathly Hallows, and all month we'll surf the Potter fan sites, scan the message boards, and read the tie-in books to argue the biggest pre-publication speculative debates.
Theory: R.A.B., who signed the letter Harry found instead of a Horcrux in Voldemort's cave, is Sirius Black's brother Regulus.
The argument for: Whoever it was who stole that Horcrux must have been a powerful wizard, and one who (like Dumbledore) knew a great deal about Voldemort's quest for immortality. Regulus, who joined the Death Eaters before trying to back out and getting killed by Voldemort (or on Voldemort's orders), would certainly fit the profile. And in the Dutch edition of Harry Potter and the Half-Blood Prince, the Black family name is translated as "Zwarts," and R.A.B. becomes R.A.Z.
The argument against: There isn't much of one. Many have proposed alternatives, like "some relative of Burkes from the Dark Magic shop" or "Dumbledore's brother Aberforth, if you mess around with his initials a little." One essayist even proposed that R.A.B. isn't a name at all but an acronym. But no one has a solid case to make.
Our take: It's Regulus. No one else fits. Unless that's what J.K. Rowling wants us to think…
Wade into the debate: Over on MuggleNet's message boards, there's a thread for the vocal minority who still don't believe R.A.B. is Regulus Black. Post your rampant speculation as to why R.A.B. is actually Dumbledore From the Future there.