Harry Potter Frenzy - Aberforth Dumbledore, half the mystery revealed

Aberforth is the bartender at the Hog's Head in Hogsmeade, as revealed by J.K. Rowling in an interview. She first provided hints of this in Harry Potter and the Order of the Phoenix, when Harry noted a strong odour of goats in his bar and felt that he looked familiar (he had been shown a photo of Aberforth earlier in the book, and as Albus Dumbledore's brother, he looks like him). In Harry Potter and the Half-Blood Prince, Harry sees him in the company of Mundungus Fletcher, who appeared to be selling stolen goods from Grimmauld Place. This, despite Sirius telling Harry that Mundungus has a life time ban from the Hog's Head for offending Aberforth and that Aberforth has a long memory. The last we've seen of him is at the end of Harry Potter and the Half-Blood Prince attending his brother's funeral.

Although not referred to as Aberforth, the bartender of the Hog's Head is mentioned occasionally by Dumbledore as a source of information. When Voldemort returned to Hogwarts to ask Dumbledore for a teaching job some ten years after leaving school, Dumbledore shows that he has unexpected knowledge of Voldemort's meetings at the Hog's Head. He explains his knowledge by saying that he is 'friendly with the local barmen'.

Sybill Trelawney told Harry that when she was being interviewed for a job at Hogwarts at the Hog's Head, and just after she had (unknowingly) spoken the prophecy involving Harry and Lord Voldemort to Albus, that they were interrupted when the 'rather uncouth barman' burst in the room along with Severus Snape. Severus claimed to have become lost on the stairs.

If the theory that RAB is Regulus Black and the missing horcrux is indeed the locket from Grimmauld Place (mentioned briefly in the Order of the Phoenix), then it is possible that Aberforth is now in possession of Slytherin's locket due to the dealings he had with Mundungus Fletcher in Half-Blood Prince.