Kathy Sierra writes an illuminating piece about how unexpected surprises produce feelings of delight. Even the subtle out-of-context surprise can trigger some neurons and brain chemistry. A reference to a movie slipped into the dialog of... another movie.
(Watching E.T. the Extra-Terrestrial this weekend, I noticed E.T. chanced upon The Quiet Man starring John Wayne and Maureen O'Hara. In a delightful scene, E.T. watches Wayne kiss O'Hare; in turn, E.T.'s feelings are transferred to Elliott who kisses the school girl with a crush on him.)
Kathy provides a variety of examples where companies create delight through unexpected context. In conclusion, within the context of the article, Kathy supplies the tips to create out-of-context user delights.