Simple Ansible playbook for demonstrating spinning up a VM on DigitalOcean

gistfile1.txt

# Invoke this twice:

# 1. Invoke with an inventory file containing nothing but 'localhost'
# 2. Invoke with digital_ocean.py inventory plugin as the inventory

# ansible-playbook -i localhost.inv equestria.yml && ansible-playbook -i digital_ocean.py equestria.yml

# Ansible 1.4 (github devel) required for unique_name option

- hosts: localhost
  connection: local
  sudo: no
  gather_facts: no

  tasks:
    - name: Add SSH key to DigitalOcean account
      digital_ocean: >
        state=present
        command=ssh
        name=laptop_ssh_key
        ssh_pub_key='{{ lookup('file', '/home/blocke/.ssh/id_rsa.pub') }}'
        client_id={{ lookup('file', '/home/blocke/.do-client_id') }}
        api_key={{ lookup('file', '/home/blocke/.do-api_key') }}
      register: do_ssh_key

    # Ubuntu_13.04_x64 512MB VM in New_York2
    - name: Create DO VM for Equestria
      digital_ocean: > 
        state=present
        command=droplet
        name=equestria
        region_id=4
        image_id=350076
        size_id=66
        wait=yes
        unique_name=yes
        ssh_key_ids={{ do_ssh_key.ssh_key.id }}
        client_id={{ lookup('file', '/home/blocke/.do-client_id') }}
        api_key={{ lookup('file', '/home/blocke/.do-api_key') }}


- hosts: equestria
  user: root

  tasks:
    - name: Install packages
      apt: name=nginx state=present

    - name: start nginx
      service: name=nginx state=started enabled=yes

what does 'register: do_ssh_key' actually do and where does it come from?
I just get the error;

fatal: [localhost] => One or more undefined variables: 'do_ssh_key' is undefined