The Fourth Wall

I was reading an article in THE CHILDREN'S WRITER about how more and more plays are breaking through the "fourth wall" between the actors and the audience. The trend began when Peter Pan first asked the audience, "Do you believe in fairies? If you believe, clap your hands."
It seems to me that many books for children also broach that fourth wall by having the author directly address the reader. Kate DiCamillo does this brilliantly in The Tale of Despereaux.
Do any of you know of other books that break through the fourth wall?
What effect does this have on the reader's experience of the story?
When should a writer broach the fourth wall? When shouldn't s/he?
It seems to me that this technique is most successful when the character or narrator speaking to the reader has a distinctive persona.
I'd love to hear your thoughts on this.