How to do Test Driven Development (TDD) in Drupal?
Since Mark's blog is offline, I'll mention some of the tools his team has implemented:

Functional testing: Selenium
Unit testing: Simpletest
Build server: Jenkins
Performance benchmarking: XDebug + Cachegrind

In the two years since I asked this question I've seen some additional tools gain popularity to the TDD scene. Nowadays when you speak of Test Driven Development (in a Drupal context, of course) there's two sides to the same coin — front-end testing, and back-end testing.

Here are two presentations that stand out from the latest Drupalcon Portland 2013 representing this matter:

Development, By The Numbers, backend testing.
Automated testing with Jasmine and PhantomJS, frontend testing.

The first presentation is not related to unit or functional testing (strictly speaking), it's more about tools to measure code quality. Nevertheless, I feel it's somewhat related to the topic.