I was at the RoadMap seminar a couple years ago and he gave a glimpse of his system which was in the Palm Desktop at the time. I was amazed, and I mean utterly amazed, at how simple it seemed. But it was complete!
They also have a mock up of their suggested paper planner in a 3-ring binder. I had the same reaction. Really, really simple.
David went from a Time Design paper system to the Palm Desktop in conjunction with the Treo 700p, which I believe he still uses for his personal system. For groupware, the David Allen Company uses Lotus Notes.
Any idea how David sets up palm for GTD. I have recently gone back to palm after having used it years ago. I find for calendar and contacts it works best. I have committed to going vanilla this time instead of getting a bunch of programs that are supposed to make you more productive. I use a mac so i wanted to set up with Missing Sync for ICal and address book but using palm desktop is probably the most vanilla you can go.
For task and contexts i find that paper works the best! I am in the process of setting up a simple 3-ring binder with tabs for simple contexts @computer, @work, @home, ......I have been tinkering for some time so we will see how it goes as i prepare to go back as a teacher for another school year!