The user management is handled by Devise
https://github.com/plataformatec/devise
and the user confirmation is part of this.
If you check through the forks Fulcrum on Github I think a few people
have done this. I'm not sure if there would be a sensible way to turn
it on and off through a configuration option, and I'd like to keep it
on as the default, it helps a lot with ensuring people have set the
right email address when signing up for notifications and password
resets.
Malc