Box Office Mojo is currently listing November 21st 2008 as the release date for 'Half-Blood Prince'. David Heyman, the producer, has previously alluded to when the sixth film might appear although nothing is official, especially since the movie doesn't even have a director yet.

Source: The Leaky Cauldron

UPDATE: WB have confirmed that November 21st 2008 is the release date for 'Half-Blood Prince'.