I have successfully setup the newsletter subscription for guests, so that if a guest signs up for the newsletter, he gets the newsletter confirmation request email where he has to click the confirmation link. This "double opt-in" is needed for every newsletter-signup by law here in Germany.
The problem is with customers who are already registered: they can subscribe in their account to the newsletter but here there is no double opt-in. So after checking subscribe they get immediately subscribed to the newsletter without getting the confirmation request email.
I have found only old tutorials on how to change this to double opt-in for registered customers, but these tutorials do not work anymore with my current version of Magento (1.5).
So how do I change the function "subscribe" in app/code/core/Mage/Newsletter/Model/Subscriber.php
to get this behavior?