Saw the Prince Caspian movie last night. They took a considerable number of liberties with the original story. I'm not complaining, mind you, especially since the original story was not as engaging as the other tales in C.S. Lewis's Narnia series. Just for the record though, the major changes included the following:
There was a James Bond-esque attempt to infiltrate Miraz's castle and take the fight to him. This ends with a battle in the castle itself and a tragic setback.
Romantic sparks between Susan and Caspian. I don't normally appreciate shoehorning a romantic subplot into a classic story, but in this case, I didn't mind. Susan was supposed to be the beauty of the Pevensie group, and so it made sense that they'd show a couple of guys responding to her in that manner.
Caspian blows Queen Susan's horn when he falls off his horse as he escape from Miraz.
The witch/werewolf scene takes place toward the end of the movie. The way this scene unfolded was changed as well.