I really like Pocket Informant as a Outlook Mobile replacement. I have also used ListPro (www.iliumsoft.com) for years. It's a very flexible list/outliner program. They have a list exchange which contains a template for GTD using ListPro (http://www.iliumsoft.com/download/lists3/GTD2.clf).
As a capture tool, I just recommended Voice Minder (http://www.pocketinformant.com/?p_id=voiceminder) in another thread. It records your voice note and emails it to your inbox.