How to make passwords optional using Devise?

Question

How can I make the password optional into the registration and login when authenticating a user (if no password then it will just use the e-mail) using Devise. Although if they register with a password (or update it later) it should then be require for login. How can I accomplish this using Devise?

Note: I'm using Rails 3.0.1 and Devise 1.1.rc0