It has just been announced that Harry Potter and the Half-Blood Prince has been postponed until July 17, 2009.
According to Warner Bros. president and chief operating officer, Alan Horn, the change was only done in order to "take advantage of an open weekend in Hollywood's busy summer season."
Harry Potter and the Half-Blood Prince was originally set to open on Nov 21st of this year, but instead Twilight will be taking it's place. Twilight's original release date was set for Dec 12th , 2008. But I guess Summit thought us Twilight fans couldn't wait that long. =]
It also has been announced that the date changes for Harry Potter and Twilight are completely unrelated.