Mark has taught, written and presented on Challenge Based Learning over a number of years.
http://ali.apple.com/cbl/components.html
[Graphic licensed under terms & conditions [Graphic River - Envato Marketplace].
His resources are linked below under these blog tags.
http://ictedservices.typepad.com/icted_services/cbl/
http://ictedservices.typepad.com/elearning/challenge-based-learning/
There are also CBL video interviews here.