Anywhere I’ve worked up to now, testing was always seen as something that you implemented after the fact. Code coverage was the main driver of the testing. However this approach completely misses the input that TDD has on the design of the application. By writing applications so that they can be tested easily, they turn out to be much better designed. They are less coupled and all the dependencies are visible. Having the design emerge from the growing system is better than imposing over elaborate architecture and patterns top down.
I found a couple of good resources recently on testing and the impact it has on your code. This is a good talk by Michael Feathers on the synergy between testing and design. He shows how testing problems are indicative of design problems. Misko Hevery’s site has some great presentations and resources on how to design code that is testable.