Fox news has a new article that states another reason for WB moving the release of HBP.  It is not an official reason, just another scenario for why they could have moved the film.

Officially, there is no reason, except that “a spot opened on the summer schedule.”  That’s not much of a reason. The real story? Harry Potter star Daniel Radcliffe will be right in the middle of his sensational, highly publicized run on Broadway in the play, “Equus.” […] That’s not the image Warner Bros. wants associated with bespectacled Harry, who remains chaste and virginal.

[…] For the movie to open on Nov. 21, Radcliffe would have to do publicity entailing answering questions about blinding horses and having sex with them vs. flying around and making potions. Warner Bros. does not want this scenario. By the time “Harry Potter and the Half-Blood Prince” debuts, “Equus” will either have concluded its run or Radcliffe will have completed his contract. And there will be little mention of any horsing around.

Source: Perez Hilton