Harry Potter And The Half-Blood Prince has been moved by Warner Bros from November 21st 2008, back to July 17th 2009. The reasons given are that they feel the movie will reach a wider audience with a Summer release date, and that the writers strike left a big gap in the Summer calendar that they need to fill. However, the move does mean Potter faces more competition from movies such as Ice Age: Dawn of the Dinosaurs, 2012, and Land of the Lost. Although I get the feeling they should be more worried than Potter.
As a result of the move by Warner Bros, Disney have decided to move Bolt up a week into the November 21st date that was left empty by Harry Potter and the Half-Blood Prince.