After this feature was requested many times on the forums but I always disagreed to it, I finally got over myself and implemented an updated version of a plugin to support that.
The former user self-registration plugin serendipity_plugin_adduser as been enhanced. It can now be configured to add users that are only allowed to log in and maintain their configuration; those users cannot post entries. To support that you will need the latest changes committed to the 0.9 SVN branch.
A new event plugin serendipity_event_adduser has been added as well. This plugin takes care to forbid users posting comments if they are not logged in. Also the plugin directly offers an interface to let users register themselves.
When a new user registers, a pending account will be created for him and he will get an activation Email. As soon as he confirms a link, the author account will be created.
If you enable the feature that only allows posting comments to registered users, only logged in users (cookie needed!) will see the "Post entry" form. Others will see a message to get themselves an account. Also note that even though users may still enter their username and Email, it is resetted to their real username+email they maintain in their profile.
Of course this is a bit beta-ish once again, so I'd love to hear feedback about it.