The books turned pretty dark around Goblet of Fire (4). I'm re-watching the movies now, just finished Chamber of Secrets (2) last night, and I wouldn't be comfortable taking a kid under 10 to see it. Since the niblings who live near me are forbidden all things Potter, the point's moot.
If they want to hang on to a PG rating, they'll ax any serious nudity. I hope they're just trying to keep interest alive in the (far too long) interim between movies.
Roald Dahl was pretty dark too but I don't recall Matilda or Willy Wonka going starkers in their movies.