The iPhone, iPad, and iTouch do not have suitable built-in task lists. Fortunately, there are hundreds of thousands of third-party apps available.
Currently, Priacta recommends either IMExchange 2 (for Exchange Server, find it in the App Store) or Pocket Informant for iPhone/iPad/iTouch (for desktop sync, see www.pocketinformant.com). Neither of these is perfect: IMExchange 2 is a good app but has marginal tech support, and Pocket Informant only supports desktop WiFi sync (an Exchange sync is being considered). See Priacta’s list of TRO-capable apps at stg.priacta.com/gtd#tro for other options.