Great example on Creating a MOSS custom field type

As you know – MOSS is extensible in pretty much all directions. The product team has
done a great job on this latest version!!!
There are two elements you need to create to get this to work:

1. an XML file – tells Sharepoint what characteristics/metadata about your new field.
2. a .NET class(es) to control the fields behaviour during rendering for view, edit and
   new modes. (so you can precisely control how the control appears)

My buddy Clayton is up all hours and has nutted out a simple example – check it out here