I have been using this GTD stuff now for a short time, and have added 1 extra thing to it that helps me, I add a reference number to each item that starts with the date in number format, ddmmyy followed by a serial number. That way, that particular to do item has a unique number that I can refer to and esp. find the date of, and also it allows me to have lots of to do lists, but really only 1 master all inclusive list going into the future with a way to determine where and when the idea came from. It's easy to do. Well at least for myself, it's an improvement.
And since you'll never exceed a million things, the number won't get any bigger than 6 digits, other than the date, but for a given day, you could have a short form for the date.