According to
Casting Call Pro, five new actors have been cast in
Harry Potter and the Half-Blood Prince, some of whom might just be extras in the film.
Lord Johnpaul Castrianni is listed as playing a Death Eater.
Katy Newman-Wood plays a Hogwarts student (she had the same role in
Order of the Phoenix).
Guy Mannerings plays one of teenagers in the Slug Club.
Nina Voelker will play a new and original character named Wendy Slinkhard.
Daniel Barnett plays a Hogwarts graduate attending Slughorn's party.
We await confirmation from Warner Brothers.
UPDATE: We've got more pictures of some of the above actors. You can see them in our gallery at these links:
Daniel Barnett,
Guy Mannerings and
Katy Newman-Wood. Thanks to
UHP for the tip!