Ask HN: QA testing for Agile teams

I've read a number of papers and presentations on how best to integrate QA processes into Agile development, and have some ideas of my own, but I'm curious how other HN folks have tackled this problem.

Obviously, pervasive testing (from unit tests to Selenium) helps a lot with improving quality, but I suspect there's still some value to putting an experienced tester in front of a running instance of your application and seeing what they can break.

How have you guys worked this into the development workflow? Test stories? Periodic "feature freeze" periods where QA drives the development of bug fixes? External QA that simply submits bug reports into the product backlog?