We are testing the new Reminders application that was released with iPhone OS5. This application is designed to give you Tasks on the iPhone that will sync through iCloud to iCal, or iCloud or Exchange to Outlook. If the application passes our rigorous testing, we will let be sure to let folks know about that. It's not something we are able to officially recommend or endorse as a GTD solution at this time without further testing.
Earlier versions of the iPhone (prior to OS5) were not built with a Tasks application, so you will need to find an App that will support the GTD best practices for a list manager (e.g. sort by context, allow but not force due dates and allow related notes to be attached to the Task.) What you choose for the iPhone would depend primarily on what do you need to sync it to. The corresponding application you want to sync it to (Mac, PC or Web) would narrow down the best application for you to install on the iPhone for Tasks. The end result is that you would have a Tasks application on the iPhone-->synching through a service-->that then shares that information with a corresponding application on the web or desktop.
You may also try searching or posting your question to our public forums for recommendations from other people who have implemented GTD on an iPhone. http://www.davidco.com/forum/
