Specific DVDs go "out of print" all the time, sometimes due to poor sales and sometimes due to elaborate marketing schemes (koffDISNEY). Anyway, the point is this: If you want to own a "Harry Potter" movie on DVD, you better buy your copies before December.
From Pottersite HPANA: "Warner Bros. has confirmed to HPANA that it plans to pull all of the first four Harry Potter DVDs off store shelves by early 2007, with no announced plans for their return. "Sorcerer’s Stone," "Chamber of Secrets" and "Prisoner of Azkaban" will become unavailable after December 29, 2006. "Goblet of Fire" will follow suit on February 9, 2007.
First reported by Home Media Retailing Magazine, the move may indicate WB’s interest in stoking sales this non-Potter holiday season, or preparing for newer editions of the films on HD-DVD or Blu-Ray technology."
(Or just another set of re-issues once "Order of Phoenix" hits DVD.)