In my opinion everybody should find his own way and his own tools for implementing David's algorithm for Getting Things Done. Everybody can also modify or enhance this algorithm if these changes lead to the main goal: "mind like water" state. This state is possible only when you have reliable external to-do memory system which is comfortable FOR YOU. So David's tools list may be interesting but use the tools that are fun for you.
For example I am using Siemens ME45 GSM phone as my mobile NA container. You can say that it is difficult to synchronize with Outlook or Palm or that its organizer software is very limited but ... it is always with me, it has digital voice recorder and has memory that is much better and accurate than mine.