An interesting new editorial on Mugglenet today puts forth the theory that the famous R.A.B.--who is assumed to be Sirius's brother Regulus Black--might in fact be Sirius's great-uncle, who was also named Regulus Black.
This is one of the better R.A.B. theories I've read since right after Book 6 came out. When you look at all the names and time-lines and such (i.e. that Regulus I's uncle was Herbert Burke, potentially of Borgin and Burke's; that all involved would have been adults at the time Voldey was in power as opposed to the just barely out of Hogwarts Regulus II) it really makes some sense. (Click on the chart to make it bigger, by the way.)
There are parts I don't agree with, of course. I think all the death speculation is possibly pushing it a little--maybe Regulus I died because of what he did with Voldey's horcrux, but I don't think that was the cause of death for half the Black family.
I also wonder about the author's conclusion that Dorea and Charlus Potter aren't Harry's grandparents. I suppose they don't have to be, and indeed it would be hard to believe Sirius wouldn't mention it if James and Harry were related to him--especially when Sirius makes his whole speech about how he and Tonks are cousins, and he's related to the Weasleys and etc. On the other hand, Dorea and Charlus only had 1 son (remember, James is an only child), and it doesn't appear that Harry has tons of relatives on his Dad's side (ostensibly, they're all dead and that's why Harry lives with the Dursleys). I'm not saying I think they are Harry's grandparents, and I'm not saying I think they aren't--but I do think it would be fun to find out one way or another. Only 3 days left to wait!