Prototyping can be very difficult with TDD - when you're not sure what road you're going to take to a solution, writing the tests up-front can be difficult (other than very broad ones). This can be a pain.
Honestly I don't think that for "core development" for the vast majority of projects there's any real downside, though; it's talked down a lot more than it should be, usually by people who believe their code is good enough that they don't need tests (it never is) and people who just plain can't be bothered to write them.
