Hey MFA, I love that book. There is a website called Vitalist.com that basically turns his whole method into a website. ticklers etc.. I'm not sure if David Allen is involved with this website or not. But the purpose of the site turns his book into actionable form.
You can enter in all your "actions" and then label them etc. etc. It takes a little bit of time to learn the website, but once you do, it's great. I check my list every day. When I make my lists, I dont go as detailed as he describes in the book. I find that takes too much time, labeling everything. But it keeps me on track. because i know if something is on that list, i wont forget it.
If you try out the site and have any questions about how to use it, feel free to ask...I spent a lot of time learning it a while back. If you learn how to use it, you will totally understand the GTD system.
One new feature they added on the website, that i really like but took getting used to, is that once you cross something off your list, it's gone, you can never get it back and look at it again. I guess that it's supposed to reinforce the whole principle of O.H.I.O. "only handle it ONCE"...