Continuing on the topic of unit testing, this quote sums up the difference between unit testing and test driven development:
It's important to remember that TDD is about design. It is not a synonym for 'unit testing'.
Having the design as a goal of TDD is definitely a paradigm shift to those folks that are used to unit testing. Even if you're an expert at unit testing, employing it at the beginning as a design step will take some getting used to.
But let's not even talk about how many different code generators are killed by using TDD as your design step rather than doing your design as an ERD in the database.