The need to create test code that verifies each unit in isolation forces us to decouple the code in ways we might not otherwise think of.  Thus, testability is a force that drives the design to a less coupled state.  --<a href="http://btobshop.barnesandnoble.com/textbooks/booksearch/isbninquiry.asp?isbn=0135974445"><i>Agile Software Development Principles, Patterns, and Practices</i></a>, p. 424, <a href="http://www.objectmentor.com/aboutUs/bios/Robert%20C.%20Martin">Robert Martin</a><p>