Check out the slides from my Railsconf 2012 Lightning Talk on Speakerdeck:

http://speakerdeck.com/u/tddium/p/tests-are-part-of-your-product-railsconf-2012

I’ll be expanding on these concepts and sharing my thoughts on how developer-written tests fit into a strong engineering culture over a series of blog posts in the next weeks.  Stay tuned!