Drupal Registration Form In Lightbox With Ajax
If I recall correctly, instead of calling render() for your drupal_get_form(), call drupal_render() instead.

&lt;?php 

function ajax_register_mypage($form){
  print 'Hello World';
  print drupal_render(drupal_get_form($form));
}

?&gt;


Hope this ends up solving your issue on this one.