See my web page http://web.ncf.ca/an588/abc.html
which is a reply to David Allen's newsletter post
"Why ABC Priority Codes don't work" http://www.davidco.com/newsletters/archive/0512.html .
I find it's useful to record priority level when I write down an action. I agree with what David Allen says except his conclusion; I don't see any reason not to do write down priority levels. I find it saves me time as compared to using his system in a purer form.