Who is RAB?

I hate being dragged into a guessing game about who RAB is in the sixth HP book, Harry Potter and the Half-Blood Prince. No wait, I think I'm looking forward to the next book, due out on the 21st July 2007.

Obviously the smart money is on RAB being Regulus Black, who we know is listed on the Black family tree, and was in Voldemort's service until he realised what was going on.

Reading through the Order of the Phoenix I realised that whilst cleaning the Black household they find a "heavy locket that none of them could seem to open".

Methinks they had the locket all along!