I have been asked to create an actual HTML page / JavaScript to simulate detection of the mobile devices (iPhone / iPad / Android) using JavaScript code. This will then take the user to a different screen which asks them for their email address.
I know this answer is coming 3 years late but none of the other answers are indeed 100% correct. If you would like to detect if the user is on ANY form of mobile device (Android, iOS, BlackBerry, Windows Phone, Kindle, etc.), then you can use the following code:
if (/Android|webOS|iPhone|iPad|iPod|BlackBerry|BB|PlayBook|IEMobile|Windows Phone|Kindle|Silk|Opera Mini/i.test(navigator.userAgent)) {
// Take the user to a different screen here.
}