According to IMDB.com (and numerous HP sites),
Broadbent Lands Potter RoleHe'll be perfect!
Oscar winner Jim Broadbent is set to become Harry Potter's latest teacher after signing on to play Horace Slughorn in Harry Potter And The Half-blood Prince. In the Potter books, Slughorn is Hogwarts' potion master and the brains behind the fictitious magic school's Slug Club. The British actor tells DarkHorizons.com, "Slughorn is drawn back out of retirement because he's got some secrets Harry and (headmaster) Dumbledore need in the battle against the Death Eaters. He's quite star-struck as a teacher, and he's drawn back into the fold because he likes to notch up celebrity students."