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. You will also need an adapter to connect both devices together.

See it in action on this video:

You can find the schematics for them here: iPhone Serial Port

You can find Newton Keyboard in the namedfork.net repository in Cydia.

NewtonKeyboard is provided under the Mozilla Public License.
It contains software from the Newton Desktop Connection Library.

• You can check out the source code from subversion:
  svn checkout svn://vc.namedfork.net/namedfork/newtonkbd/trunk
  
• Or browse and download it with WebSVN:
  http://vc.namedfork.net/nf/newtonkbd/