First it was Gilad Bracha -
http://gbracha.blogspot.co.uk/2012/11/debug-mode-is-only-mode.html -
and now John Nolan -
http://stigmergist.blogspot.co.uk/2013/01/debugger-first-vm-development.html

I also saw a really neat demo of a framework for debugging system
calls. I hope to find out more about this when I quiz the implementor
in early February. I would be less vague if I could remember the name
of the framework!

frank