I have the GTD add-in because I have Outlook (the full client) at work where things are kept on the Exchange server (no PST). It works nicely on Outlook, however I do not see the add-in if I use OWA (using OWA 2000/XP not 2003). I can look in the folders that the add-in puts in (@Action, @Calls, etc.) and I can see the tasks, but the new views are inaccessible to me. The new fields (Project, Action) are not available. I can't say for certain it's unavailable; when I use OWA, I'm on a Mac, generally using Safari, though sometimes using MSIE.
Related: Our IT dept has made Outlook 2000 accessible via Citrix. The add-in is not visible nor my new fields, but my GTD views are visible, so I can manually drag an email to task list then drag a task to the action and project so that they are placed in context