…since Silverstripes‘ Userforms Module Uses jQuery for Validation you can put these lines of code in your UserDefinedForm.ss Template:

jQuery(document).ready(function(){
    jQuery('form label').each(function(){
      jQuery(this).parent().find('input, textarea').attr('placeholder', jQuery(this).html());
      jQuery(this).remove();
    })
  });

This will iterate over the labels in the form , build placeholder attributes from each one and then remove it.

If anyone knows a way how to do this in PHP, i’d appreciate a comment :)