As promised at their October 4th media event, Apple has released their new Cards app for iOS (Free, App Store Link), allowing you to create and mail custom greeting cards from your iOS device.
From Apple:
The new Cards app from Apple lets you create and mail beautifully crafted cards personalized with your own text and photos — right from your iPhone or iPod touch. Take a quick snapshot and with a few taps and swipes, an elegant letterpress card is on its way to any address in the world. Each card is just $2.99 when sent within the U.S. and $4.99 when sent to or from anywhere else. And that includes postage.
The cards app is available now through the App Store, requires iOS 5, and is a free download (Free, App Store Link).