RexWireless makes products for the Blackberry, specifically designed for GTD usage. They are called ToDoMatrix and IdeaMatrix. You could imagine them as a tree outliner of folders and tasks, and the data can be viewed based on @Context, or Priority, Due Date, Delegate, etc.
Originally Posted by LMeeker
The company has a web interface also, which is called RexDesktop. It is accessible from any browser - Windows, Linux, or Mac.
There is no need to manually sync, as the Blackberry updates the web version automatically and vice versa.
One enormous advantage of this setup is that you can lose or break your phone and have both computers crash and you will still have access to all of your data from any other computer's browser.
This is, by the way, my favorite GTD setup because it allows you to be productive anywhere you have your phone... which, for most people, is everywhere.
In fairness, I will note that there is a bit of a learning curve to the software, though it can implemented immediately while you learn all of its capabilities. My opinion is that it is worth the effort. They still offer free trials I believe.