Apple iPhone 4, Apple iPhone 4S and Apple iPhone 5, 5th generation iPod touch and third generation Apple iPad (and most likely the Apple iPad 4 and iPad mini) in order to use the app.
Will the recipient of the card be fooled into thinking that you took the time to send a handwritten card? Inkly swears that your laziness will never be detected. A version for Android is said to be on the way.
source: AppStore, TheNextWeb via Gizmodo