The webcast will be available for viewing after the live performance on demand, anytime you'd like to hear it. And the evening before (May 8th, last night as I write this) was a Webcast by Waterson:Carthy, a more traditional program, with her parents Martin Carthy and Norma Waterson, and two other blokes names Tim and Ben, whose last names I didn't catch. (I was at the Millennium Stage live to see them last night, and I'm about to go off now to see the performance mentioned above.)
The archived programs are available at http://www.kennedy-center.org/programs/millennium/archive.html, or click here. Then search for "Carthy."