Yep, according to Rope Of Silicon, Harry Potter And The Half-Blood Prince has been rated PG for "scary images, some violence, language and mild sensuality." Mild sensuality? Gosh, maybe I'll wait for the uncut 'Too Hot for Muggles' edition.
Seriously though, while it is somewhat unexpected news, I'm frankly surprised that the last two films were PG-13 in the first place. Yes there was some fantasy violence (like the second act dragon-chase), even a handful of on-screen murders (via conveniently gore-less green energy blasts), but nothing that anyone would have batted an eye about had the films retained the PG of their predecessors. In the last two films (especially Order Of The Phoenix), the violence was far more emotional than physical.
If I remember the sixth book correctly (and it has been 3.5 years), there isn't much violence until the climax, and again we're talking people shooting green energy beams at each other. There are a few deaths and one 'off screen' mauling, but again it's more of an emotional roller coaster than an violence spectacular. As it is, director David Yates has already said that he'll be trimming some of the Tom Riddle flashbacks to allow more screen time for the neglected supporting cast, so maybe some of the nastier bits got cut for reasons unrelated to ratings.
Of course, the seventh book... that one will be a hard PG-13, even if the obscene amount of carnage is relatively bloodless. But as for Harry Potter 6, chalk this up to a The Legend Of Zorro situation... same amount of violence, identical body count, a little less blood and dirt (and no heads in a jar), and you go from a PG-13 for The Mask Of Zorro to a PG for The Legend Of Zorro. Point being, don't panic. Considering that the PG-13 volumes have been the most successful sequels thus far, I don't think Warner Bros. has any incentive to further antagonize the fans by intentionally toning down the material for the last two books. Besides, if it's as 'PG' as Star Wars: Episode II: Attack Of The Clones (violence galore, onscreen murders, off screen genocidal massacres, three or four beheadings), then there shouldn't be an issue.
Oh, and spoiler-phobics, please avoid the Entertainment Weekly 2009 preview article, which casually gives away a major climactic development.
Now, about that 'mild sensuality', we can only hope for a Hogwarts threesome with Hermione, Ginny, and Luna Lovegood. And, for per wife's approval, maybe Snape can show up halfway through to deal with the devious younglings and/or join the fun -- Alan's call really.