In my Agile Australia 2011 talk I placed these in my 'hard and expensive' quadrant. These are my ‘don’t even bother trying without help’ topics. My somewhat tarnished ray of hope is that you can be agile without doing both of these, or even doing one of these. This might be a bit controversial to say to purists I guess.
To start practicing Test Driven Development or Acceptance Test Driven Development you need help from someone who has done it before, even then I would say you need several people. This is one that needs support from management to try it, and consultants to coach in the practices, also some really keen technical folk that can see the sense of it and want to give it a bash.
It’s been a pretty challenging time however it has reaped success and we are getting great results where we’ve done it, it’s still embryonic but I can see how it’s going to become mature for us. I think doing ATDD and TDD is very contextual to the software you are building; definitely easier for green field than legacy codebases.