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