Based on the well known Subscribe-to-Comments PlugIn, but now with double-opt-in feature to prevent spam.
Use this code (within the loop) in your template file, to change the place of the checkbox:
<?php show_subscription_checkbox(); ?>
Yes, just place this code snippet in your template (outside the comments form):
<?php show_manual_subscription_form(); ?>
See this (german) comment for a solution: Go to Comment