This is really really easy in Drupal 7: just use field_view_field or field_view_value. Choose the latter if you want to re-render a particular field value, or the former if you want to re-render the whole field (label and all values).
However, I couldn't figure out how to do this in Drupal 6. I resorted to implementing hook_nodeapi and changing the node content before it was rendered. This approach cannot be implemented from the theme, though, I'm looking for a theme solution.