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Sometimes, a move like this might indicate problems with the production of the movie itself (an actor drops out of the film, preview audiences didn't like the early cut of the film, the budget was escalating out of control, etc.). I would be surprised for that to be the case here, as the cast and crew are all well-seasoned, and early word-of-mouth is excellent.
Other times, a studio might be trying to avoid poor timing around an unrelated event. For example, immediately following September 11, 2001, several movies (Zoolander) were re-cut and/or postponed to avoid showing images of the World Trade Center or other potentially disturbing imagery. One particularly gossipy columnist from FoxNews.com asserts that the Warner Bros. decision to move HP6 was to avoid ugly press from Daniel Radcliffe's upcoming stint on Broadway in the controversial play, Equus, in which the actor appears nude. Unappealing as the actor's decision is, the play itself is old news--he performed it to rave reviews in London over a year ago--and unlikely to affect the performance of one of the series' most popular installments.
Still another ridiculous rumor is that Warner Bros. moved Half-Blood Prince out of the holiday season so as to avoid competition with Twilight. My own ambivalence for the vampire book notwithstanding, I'm quite sure there was room this winter for both movies, especially where they were scheduled to open three weeks apart. In this battle, I'm fairly certain that Harry would have kicked the crap out of Edward Cullen, so I don't believe this theory at all. (In an interesting side-note, Summit Entertainment has now moved the release of Twilight up three weeks to take the spot previously occupied by HP6.)
According to the studio itself: “...we are still feeling the repercussions of the writers’ strike, which impacted the readiness of scripts for other films—changing the competitive landscape for 2009 and offering new windows of opportunity that we wanted to take advantage of. We agreed the best strategy was to move Half-Blood Prince to July, where it perfectly fills the gap for a major tent pole release for mid-summer." Translation: the studio didn't have much else going on for next summer, and needed something big to help fill in the late summer gap. I can buy that...to an extent.
A quick glimpse at the studio's release calendar for the 2009 summer season reveals only one major release: the Christian Bale Terminator Salvation on Memorial Day weekend. Typically the major studios will have at least one major "tentpole" (i.e. costly potential blockbuster) for each month of the summer season (starting in May), sometimes more. So it does make some financial sense to push a sure-thing like Half-Blood Prince into 2009, where the studio can spread out some of the potential earnings for an upcoming year that looks a little bleak.
Having said that, I wouldn't be surprised if the studio's decision had more to do with the recent mega-success of The Dark Knight, which has outperformed every financial expectation, and is currently headed toward an almost unprecedented $500 million domestically. That gives the studio a little breathing room for the rest of the year, and allows them to spread the wealth of franchises into a new fiscal year.
One final (and admittedly unlikely) theory about The Dark Knight: the critically-adored comic book adaptation is likely the WB's best shot at Oscar attention when the awards season begins at year-end. Pulling Harry Potter out of the way during the holiday season could give the studio the chance to re-release The Dark Knight into theaters for the holidays (assuming they choose to hold off on a DVD release). The theatrical re-release would not only keep the Caped Crusader (and more importantly, Heath Ledger's Joker) fresh in Academy voters' minds, but could also provide an extra boost of revenue for the film, which will likely finish its current theatrical run about $80 million shy of the current domestic record-holder, Titanic. Who cares about Harry vs. Edward...the real battle here may be between Batman and The King of the World.
Update: WB President and C.O.O. has responded to fans' disappointment over the delay of HP6. Read it here.