Update 5/8/2012: Fixed To Work With Latest Release

Just paste the below code into an html snippet on a webform, and change the fieldName variable to the name of the field you want to convert to a textarea. (Use firebug to get the field name).

<script type="text/javascript">// <![CDATA[
        var fieldName = 'inf_field_Email';
        var textArea = document.createElement('textarea');
        $textArea = jQuery(textArea);
        $textArea.attr('name', fieldName);
        jQuery(document).ready(
                function(){
                          var $originalField = jQuery('input[name=' + fieldName + ']');
                          $textArea.val($originalField.val());
                          $originalField.replaceWith($textArea);
                }
        );
// ]]></script>