Drupal 7: HTML5 placeholder attributes for form items with typical jQuery fallback for older browsers.

form-placeholders.js

(function ($) {
  Drupal.behaviors.yourTheme = {
    attach: function(context, settings) {
      $('form label').css('display', 'none');
      if(Modernizr.input.placeholder) {
        // Text inputs
        var formElements = $('form').find('input, textarea');
        $.each(formElements, function(index, value){
          if($(this).is('input')){
            var label = $(this).siblings('label').text();
          }else{
            var label = $(this).parent().siblings('label').text();
          }
          $(this).attr('placeholder', label);
        });
      }else{
        var formElements = $('form').find('input, textarea');
        $.each(formElements, function(index, value) {
          if($(this).is('input')){
            var label = $(this).siblings('label').text();
          }else{
            var label = $(this).parent().siblings('label').text();
          }
          $(this)
            .val(label)
            .focus(function(){
              if($(this).val(label)) {
                $(this).val('');
              }
            })
            .blur(function(){
              if(!$(this).val()) {
                $(this).val(label);
              }
            });
        });
      }
    }
  }
})(jQuery)

The reason textareas are handled differently here is because Drupal renders them with an extra wrapper which puts the label one level up.

this works perfect for text-fields and text-areas. My problem is, that your script also hides labels from checkboxes and radio-buttons, which makes them unusable!