I wrote a blog post describing TDD and Continuous Integration and how TeamCity fits into the equation, http://javablogs.com/Jump.action?id=330660
Here's an excerpt:
"I recently heard from someone in the industry that around 30-40% of teams are using test-driven development, but only 5-10% of teams are using continuous integration servers. I'm not sure how reliable those numbers are, but that's not important; what is important is the general picture: Even the people who seem to understand the benefits of test-driven development don't seem to understand the benefits of continuous integration. I found that very puzzling, especially because I see continuous integration (CI) as a natural progression of test-driven development (TDD). They are, after all, part of the same trend of agile software development leading towards faster and faster feedback."