There is a new program for GTD on the Mac called Task Paper (http://hogbaysoftware.com/products/taskpaper) from Hog Bay Software.
It's a simple little thing for keeping lists with project and context views. The data file is plain text. Backups, etc. are as simple as saving the file under a new name. It's the sort of elegant software I'd been looking for.
I really admire it's simplicity. You can't waste time frittering with it. As an iPhone user it integrates well into my portable carry needs. I have a special email account for my iPhone. As the Task Paper lists are plain text, I can quickly paste them into an email and send it to my phone so I can carry it with me (I actually do two emails, one just for @errands and a second for the entire GTD list).
For those interested, give it a look. I've no connection with the company.