I can't believe they're not merging Prince Caspian and Voyage of the Dawn Treader. I was expecting them to merge these two and possibly leave out A Horse and His Boy.
Nothing happens in Prince Caspian. The Pevensies return, take a billion years to figure out where they are, listen to the story of Prince Caspian running away from home, walk to Caspian, have a brief scuffle with some traitors, then have one major fight. Then Aslan runs around destroying all aspects of civilization that the author didn't like. Caspian is crowned King. The end.
Despite these misgivings, the first movie did a good job of extending and merging scenes to fit the time limit and build suspense without taking anything important from the plot or obviously padding it. I think they'll do okay with Prince Caspian.
Still, merging a book or two would ensure the series doesn't run out of steam. Sure, everyone loves The Lion, the Witch and the Wardrobe, but I think most people haven't even heard of The Silver Chair or The Last Battle and won't be too thrilled about all their favorite characters stepping aside for new ones. I've never felt it's a problem, but it does make the movies less--what's the term? Accessible?