Ever since DrupalCon Boston, unit testing has become a big topic for development - it's baked into Drupal 7, and patches should have tests to show what is being fixed. It's just as important in Drupal 5 and 6.

I plan on talking about using simpletest as you develop to prove that the code you're writing is correct as you write it, and to keep it correct as you fix bugs and add functionality. I'll also go over how to extend SimpleTest's functionality with some simple Object Oriented Programming to make it easier to test your module.