Hex offers iPhone ticketing app for Heathrow Express
The app, called Heathrow Express, allows users to peruse available train itineraries, and purchase them instantly using saved billing information. It only requires your card's 3-digit passcode, so you don't have to worry about unauthorized access to your billing information. The app will then save all your itineraries for your records.
Heathrow Express has been available for the Android OS since December, but its only just been made available for iOS and BlackBerry. There are similar electronic boarding pass options for many airlines, but they operate with varying levels of success. The American Airlines E-Ticket, for example, is sometimes hard to scan. NFC (near-field computing) should remedy some of these paperless solutions.
source: iTunes via The Register