Is TDD against the Agile Process?

Test Driven Development (TDD) is a useful output of the Agile camp but some suggest that it raises a conundrum. TDD requires that a unit test be written for a class before the class is written. It might be thought, then, that the class firstly has to be "discovered" and secondly defined in sufficient detail to allow the write-test-once-and-code-until-class-passes model that TDD actually uses. This would be actually counter to Agile approaches, particularly (so-called) Agile Modeling, where developers are still encouraged to code early, with light design. However, to get the claimed benefits of TDD a full design down to class and responsibilities (captured using, for example, Design By Contract) is not necessary. This would count towards iterative development, with a design locked down, but not iterative design - as heavy refactoring and re-engineering might negate the usefulness of TDD.