Custom callback notification with Rails 3

User.rb

class User < ActiveRecord::Base
...
def self.new_sitter(user, params)
    if user.parent?
      transaction do
        sitter = create! do |u|
          u.sub_type   = 'sitter'
          u.first_name = params[:first_name]
          u.last_name  = params[:last_name]
          u.email      = params[:email]
        end
        notify_observers :new_sitter_created
        sitter
      end
    else
      logger.info "User: #{user.id}, #{user.to_s}, with sub_type 'sitter' attempted to create a User with sub_type 'sitter'."
      nil
    end
  end
...
end

UserObserver.rb

class UserObserver < ActiveRecord::Observer
  def new_sitter_created(user)
    #do something
  end
end