David Heyman has done a new interview with IESB.net where he discusses the Deathly Hallows split and his other films.  I have put the full part where he discusses HP if you click MORE.

Do you think it’s the end for the Harry Potter franchise and characters on the big screen after the last book?
Heyman: I would not be involved in a film based on Jo’s characters without Jo’s stories, so my sense is yes, unless Jo magically writes something else Harry Potter that is not a Harry Potter tale, not Tale of Beetle the Bard or the books on Quidiche, if she wrote something like that maybe but I am not sure she has any plans to do so.

(i enjoy how they spelled Quidditch )