application.js

jQuery(function($) {
  // Fallback functionality for HTML5 input placeholder text
  if (document.createElement('input').placeholder === undefined) {
    $('input[placeholder]').prompted_text_input().blur();
  }
});

prompted_text_input.jquery.js

jQuery.fn.prompted_text_input = function () {

  return this.each(function() {
    $(this)
      .bind('blur', function () {
        $this = $(this);
        if($this.val() === '') {
          $this
            .val($this.attr('placeholder') || '')
            .addClass('showing_prompt')
          ;
        }
      })
      .bind('focus', function () {
        $this = $(this);
        if($this.val() === $this.attr('placeholder')) {
          $this
            .val('')
            .removeClass('showing_prompt')
          ;
        }
      })
    ;
  });

};