I have the pleasure of speaking at WordCamp Denver this year on the subject of Unit Testing Plugins in WordPress.

Your plugin is broken; do you know why? In this session, Ben will show you some simple principles to help you integrate unit testing in your plugins that will help you find and eliminate bugs before they happen.

View my slides

Here is the most bestest PHPUnit Testing with WordPress tutorial on the whole interwebs

Additional resources

https://github.com/blobaugh/wordpress-plugin-tests
http://en.wikipedia.org/wiki/Test_fixture

http://en.wikipedia.org/wiki/Acceptance_testing
http://en.wikipedia.org/wiki/Integration_testing
http://www.youtube.com/watch?v=J4dlF0kcThQ
http://wordpress.tv/2011/08/20/nikolay-bachiyski-unit-testing-will-change-your-life/
http://wordpress.tv/2013/08/05/alison-barrett-lessons-learned-in-unit-testing/
http://jenkins-php.org/
http://en.wikipedia.org/wiki/Test_fixture
http://net.tutsplus.com/tutorials/php/mockery-a-better-way/
http://phake.digitalsandwich.com/docs/html/