This app will allow you to use an iPhone or iPod Touch as a keyboard, to input text on a Newton running Newton OS 2.0 or later.
It contains software from the Newton Desktop Connection Library.
It connects to the Newton using the serial port on the dock connector, which is
available on /dev/tty.iap on jailbroken devices.
There is a schematic for the adapter cable, using a MAX3232 level shifter.