I've been using an interesting product (free download) for Outlook for the last few weeks that does a good job implementing GTD within Outlook. It's called jello.dashboard (current version is 2.55).
It replaces the Outlook Today page with an incredibly functional view of all your tasks, contexts, projects etc. It utilizes the Category field to accomplish what it's doing. As long as you create new tasks within the page it will correctly set up the coding in the Category field.
You can create any number of contexts and projects and link tasks to them. You mark tasks as a Next Action and they will show up in a Next Action view.
It was extremely easy to set up and get to using it. Right now the only drawback is it won't work when disconnected from the Exchange server, but I've gotten around that by printing up a "Master List" view that I take with me if I'm going to be disconnected.
It's a very well thought out piece of software. If you rely on Outlook you should take a look. You can find it at http:\\jello.wordpress.com