I just read about this on my cousin's blog and got this info off the Stephenie Meyer site:
Movie News Flash!
So, many of you have heard that the release of the sixth Harry Potter movie, Harry Potter and the Half-Blood Prince, has been moved from this Thanksgiving to next summer. First and foremost, please know that this schedule change has absolutely nothing to do with Twilight, me, or Summit Films (so enough with the imdb death wishes, okay?). This is Warner Bros. decision, and it was not motivated by anything Twilight-related.
Now for the good news! Though we're all sad to have to wait for Harry Potter, this open spot at the theater creates a cool opportunity. The good people at Summit were thrilled to let me know that now Twilight fans are going to get their movie three weeks earlier than scheduled. That's right—Twilight will be released in theaters November 21st! Let the merry-making commence!