Say you want to connect your Arduino to Android device – what options you have? Practically several that require some expense like using IOIO board, or some Bluetooth, WiFi shields. Consider another method which practically is free. All you need is several turns of thin wire.
Many android devices have magnetometer which serves as electronic compass. So why not to use it as data entry point. Transmitting coil is simply attached to Arduino digital pin using flyback diode and current limiting resistor. Using digitalWrite() function we already can send data bits. Data reception in Android side seems to be very slow, because of lag between magnetometer reads. Joe was able to send data at 7 bits per second rate. If speed isn’t an issue why not give a try.