I Thought Noone Could Apparate Inside Hogwarts. How Does Dumbledore?

"One day you'll read Hogwarts, A History and perhaps that will remind you that you can't Apparate or Disapparate inside Hogwarts," said Hermione in Harry Potter and the Order of the Phoenix.

How many times has Hermione mentioned this bit of trivia throughout the Harry Potter series? I have no idea. But, by the time Harry Potter and the Half Blood Prince is drawing to a close, this should seem like an obvious, immutable fact.

How, then, does Dumbledore Apparate himself and Harry straight out of Hogwarts to go to the cave in search of Voldemort's horcrux? It turns out this is a clear change of circumstances between the novel and the movie.

In the Novel...

In the novel, Dumbledore tells Harry that they are going to search for one of Voldemort's horcruxes. Harry puts on his invisibility cloak and they sneak out of the castle to go to Hogsmeade.

On the way, Dumbledore and Harry pass by Madam Rosmerta, the owner of the Three Broomsticks. They tell her that they're heading further on down the road to the Hog's Head. Once out of sight, Dumbledore and Harry Apparate out of Hogsmeade to the cave.

After obtaining what they think is the Horcrux, Harry and Dumbledore Apparate back into Hogsmeade, and Madam Rosmerta lends them some broomsticks to get back to the castle. They also notice the Dark Mark over the Astronomy Tower. When they appear back at the Astronomy Tower, Draco Malfoy runs in and the story converges (for the most part) with the movie version.

In the Movie...

In the movie, Dumbledore simply Apparates out of the Astronomy Tower. He says that there are certain advantages to being the Headmaster.

After dealing with the horcrux, they also Apparate straight back to the Astronomy Tower. They never set foot in Hogsmeade, nor do they meet and talk with Madam Rosmerta.

Why the Difference?

This simplifies the plotline somewhat.

In the novel, Madam Rosmerta already has a role to play. She has been placed under the Imperius Curse by Malfoy. She gave Katie Bell the cursed necklace (read more about this difference between the novel and the movie). She also sends word to Malfoy that Dumbledore has left the castle.

By this point in the movie Madam Rosmerta has already been edited out. Malfoy gave the cursed necklace directly to Katie. There was no existing tie-in to Rosmerta in the movie's plotline, so it would have been strange to bring her into the assassination plot at this point.

In an effort to simplify the plot to fit into a reasonable movie timeframe, the sideplot involving Rosmerta was completely edited out. As such, there a trip to Hogsmeade seemed unnecessary, and a bit of Hogwarts lore was bent to allow Dumbledore to Apparate straight out of the castle.