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 onscreen 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.