ive got solid evidence that R.A.B. is none other than regulus black...
here's why..
this is a extract frm a rediff article..
for d full article.here's d link
http://www.rediff.com/movies/2006/dec/22harry.htm
Reports say it took the keenest Pottermaniacs 40 minutes to unlock the door on her web site. The Internet is buzzing with speculation. Why 'deathly' hallows and not 'deadly' hallows? Are 'hallows' a reference to Halloween, when Voldemort murdered Harry's parents? Links are flying on the forums and message boards. How Rowling said, 'no comment' when asked if anyone else was present when Voldemort killed Harry's parents and tried to kill the boy wizard. How R A B, the initials of the mysterious character who is also hunting the Dark Lord's soul secrets, changed in the Dutch translation of the novel, where Regulus Black is called Regulus Zwarts and the initials read R A Z. In the Norwegian edition, apparently, Regulus has the surname Svaart, and the initials R A S. In the Finnish translation, I can tell you with knowledge gleaned from wikipedia, Regulus Black is called Regulus Musta, and the initials are R A M.