You may not have noticed, but the bottoms of In-N-Out Cups and Burger Wrappers cite biblical verses. This is an article from Snopes.com that details the origins of this practice.
Kids, don't try this at home. Frightening pictures of a group of 8 friends who decide, on a whim, to order a 100-patty burger from In-N-Out. Oh the humanity!!!
An article from Quick Service Restaurant Report that examines In-N-Out's success. Also has a more complete list of the "secret menu" than In-N-Out's official site.