Update 2/16/2012: I’ve posted this to an article on the Devise wiki. It’s likely that the article will continue to be updated by the community on GitHub. If this article appears out of date, refer to the wiki.
One of the challenges of working with Devise is that you may end up introducing functionality into the authentication layer long after initial requirements have been built.
I recently needed to add email notifications for user account creation. That worked fine, but all of the existing user accounts were left unconfirmed.