If you are yet to decide on your productivity tool of choice I have some bad news for you! Nirvana have finally released their iPhone app for there excellent GTD/Todo app.
The smooth look and feel of the web app has been carried across to the iPhone app and it works really well, it really is a mobile extension of the web site. Syncing is fast and smooth (pull your lists down to sync) and it looks plain lovely!
You can download it now direct form the Apple App store.