How to access Field-settings in the widget formatter
You can take number.module from the CCK package as an example; it allows prefix and suffix values in the field settings that are rendered in the formatter.

In theme_number_formatter_generic() the field settings are retrieved using $field = content_fields($element['#field_name'], $element['#type_name']);. The values are then taken from the $field array, sanitized and added to the output.