Credit:
These include syncing all of your actions on the iPhone or iPod Touch with the Web version of Remember the Milk; organizing everything by list, Smart List, tag, location, or time; adding notes to tasks, see tasks located nearby (by using iPhone's GPS); search using advanced search operators, and several customization features.
Bear in mind that the iPhone app is available only to Pro users of Remember the Milk, which means paying a fee of 25 dollars per year.