Some days ago, I was watching this video on InfoQ, where James Coplien and Robert C. Martin were discussing about some undesired side effects of TDD, particularly on the architecture side. One of the key point was that testing code increases the overall weight of the code base, making it harder to eventually refactor the architecture.
Another interesting issue presented was that TDD doesn't necessarily enforce testing all the possible boundary conditions, but often ends up in a sort of heuristic testing, which is less effective that testing based on a design-by-contract assumption.
Honestly, TDD book put a lot of emphasis on efforts to remove duplications, also between production and testing code, but I have the impression that this portion of the message is often lost by test writers. I've got some ruminations on the topic that will probably make up enough stuff for some more posts in the following days.