Having a play with mongomapper and wanted to get Clearance working with it. Seemed to just be a simple case of doing the following in my user model.
My full user model ended up looking like this:
key :email, String
key :encrypted_password, String
key :salt, String
key :confirmation_token, String
key :remember_token, String
key :firstname, String
key :email_confirmed, Boolean
key :lastname, String
Perhaps if I have time, I will look into adding attr_accessible support to mongomapper myself.
Update 13/03/2010: I believe the dependency on attr_accessible in clearance has since been removed. So this small hack is most probably defunct.