There was an article posted somewhere in the last 50 or so posts. The writer thinks that since it takes 2 people to get to the locket, maybe the initials are that of 2 people. Regulus and someone else? When I first read the book, I thought that maybe they were Amelia Bones' initials. RAB would work for Regulus and Amelia and the B from both their last names. However, they couldn't both go in the boat, I don't think, unless one was underage. Of course maybe the 2 of them only measured as one great wizard even when combined. Someone thought That Kreature was with him because he would probably not register as a person in the boat.
The one problem with Black being RAB is that Hermione should have discovered that when she checked. There should have been a record of his middle initial. That means that he would have given himself a nickname, like "Avenger", that he started using, which in this series seems to be a regular occurence.
Now if Amelia was actually a middle name, would Hermione have discovered her first name started with "R", but it wasn't used? Probably, official records would list the given name.
So it's likely a full or partial pseudonym.
TS
Mugglenet has an article discussing this.
But I'm really not so sure that you need two people. Remember the "find the antidote" chapter, where Hermione goes to all the trouble of separating the potion ingredients, but Harry simply grabs a bezoar?
Seems to me that RAB would need only a bezoar, and not an accomplice. And Harry will need to start carrying a couple of those little guys around in his pocket.
BTW ... I hope everything went OK for you the other day.