The Associated Press has a new article online where they state a reason for the HBP date change:

“Harry Potter and the Half-Blood Prince” now is opening Wednesday, July 15, two days earlier than its previous release date, Warner Bros. head of distribution Dan Fellman said Tuesday.

That gives the movie a head-start on the weekend during what’s shaping up as a highly competitive summer season for Hollywood, which has major franchise films opening virtually every week.

“Every day the kids are out of school in summer is just like an extra Saturday,” Fellman said.