I use ciphers and codes in my Computer Science classes as discussion topics and lab topics.  There are many different types of ciphers.  Codes are a big part of our everyday life and have been a big part of our history.  Students relate to these topics and enjoy discussing them.
I presented a Model Classroom session at the TCEA State Convention on February 11 over Ciphers and Codes.

Ciphers
Substitution Ciphers
Transposition Ciphers

Ciphers and Codes Slides
Ciphers and Codes Lesson Plan
Youtube – National Treasure Secrets and Codes

Ciphers and Codes materials will be included in the new 2010-2011  A+ Computer Science Curriculum which contains labs with solutions, slides  with examples, quizzes, tests, and related materials.