Boy I waited for this for some time now. The application I relied on heavily for utilizing public transport services in my travellings is now available for the iPhone. It’s MetrO and you can grab it for free from the AppStore.
Bare in mind that MetrO doesn’t sport fancy maps like some other similar apps (iPhone version relies on Google Maps, though), but it will guide to you to the desired station or point-of-interest without that. Simply select the starting point (station) and the end point and read easy-to-understand instructions provided on the screen.
An array of cities is supported, meaning you won’t need any other metro/tube/subway/tram application running on your iPhone. AppStore link is here, and once you grab the app, download all the cities you could ever possibly need.