That's not quite true.
While Peter is a big fan of eProductivity and I appreciate the unsolicited promotion, the real credit goes to Lotus Notes and its ability to replicate information across Windows/Mac/Linux computers seamlessly.
I've just posted a comment that explains how David Allen uses Lotus Notes to create a single trusted system across all of his computers and mobile devices.
Read: Managing GTD systems on two machines and David's Lotusphere greeting
Update: Apparently my comment son GTD Times have not been approved yet, so here's what I posted:
As David Allen’s technologist for 15 years, I can share that David uses Lotus Notes, from IBM, to solve this challenge and it works very well. with Lotus Notes, he can maintain his projects and actions in a single trusted system that replicates across ALL of his computers, including his Mac, PC, and mobile devices (e.g. Treo).
If David wants to work on his PC, he can pick up his ThinkPad and everything will be there. If he wants to play with his shiny new Mac Air, he can pick that up and all of his information will be there. And, if he’s on the run, he takes his Treo with him and his stuff is there.
In short, he simply picks up whichever tool he prefers and gets things done.
Lotus Notes makes it seamless.
In addition to Lotus Notes, David also uses and recommends eProductivity (http://www.eProductivity.com), an optional tool that makes GTD in Lotus Notes easy. You can start with basic Lotus Notes and follow the GTD Implementation guide to create a very nice system.