Programmers in XP create unit tests. That's it. Who are
going to be the people that test all the different
configurations and failure modes of the entire system?
It's a QA group. 

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BrianMarick has been looking at this question, too. See http://www.testing.com/agile/ and the agile-testing mailing list.

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See CombiningXpWithQualityAssurance.