Many ipad sproofs in these days. This one Arduino based. The interesting thing is the merchandising of old typewriters. Vintage revenge.
The Typewriter Dock, seen in the video above, is an even better version. It holds an iPad in its carriage whilst simultaneously inputting typed letters. All it needs is a Bluetooth component to replace the cable, and a writing app that can use the accelerometer to detect a carriage return and move you to a new line. Ding!
Inside there is a sensor strip under the keyboard which detects the key-presses that hit it, and this pulse of electricity is then passed on to an Arduino circuit-board whereupon it is translated into a standard USB key-down event. All you need to do is plug it in and type.