SnitchSeeker is reporting 2 new cast members being selected for Half-Blood Prince.

Tom Moorcroft has been listed to play Regulus Black in Harry Potter and the Half-Blood Prince. It is unknown in what context Tom will play Regulus since the character was never shown in the sixth book.
Actor Charlie Bennison has apparently been cast as Sanguini, the vampire who had been invited as a guest to Professor Slughorn’s Christmas party in the sixth book.

Please treat this as a rumour until we get confirmation from WB!