nested form params in rails with simple form

I'm trying to get a nested form working to create an association between a user and their address in one form.

The issue I"m seeing is that whenever validation fails, the inner address fields form is not repopulating, or showing the validation errors.

Here's the form:

<%= simple_form_for(@user, :url => register_and_checkout_path, :html => {:class => "form-horizontal clearfix"}) do |f| %>
<p>New Member &amp; Guest</p>
<small>Mandatory fields marked*</small>
     <%= f.error_notification %>
     <%= f.input :is_checkout, :as => :hidden, :input_html => { :value => true } %>
      <%= f.input :first_name, :input_html => {:class => "input-xlarge"} %>
      <%= f.input :last_name, :input_html => {:class => "input-xlarge"} %>
      <%= f.input :email, :input_html => {:class => "input-xlarge", :placeholder => "[email protected]"} %>

     <%= simple_fields_for (:address) do |a| %>
        <%= a.input :phone1, :label => "Contact number", :input_html => {:class => "input-xlarge"}  %>
        <p class="uppercase">Your Address</p>
        <%= a.input :address_1, :label => "Street Address", :input_html => {:class => "input-xlarge"}  %>
        <%= a.input :address_2, :label => false, :input_html => {:class => "input-xlarge"}  %>
        <%= a.input :address_3, :label => false, :input_html => {:class => "input-xlarge"}  %>
        <%= a.input :suburb, :input_html => {:class => "input-xlarge"}  %>
        <%= a.input :state, :input_html => {:class => "input-xlarge"}  %>
        <%= a.input :postcode, :label => "Postcode/zipcode", :input_html => {:class => "input-xlarge"}  %>
        <%= a.input :country, :priority => [ "Australia", "New Zealand", "Unites States", "United Kingdom" ] %>

     <% end %>
        <p>Become a valued member</p>
        <%= f.input :password, :input_html => {:class => "input-xlarge"}  %>
        <%= content_tag(:button, :class => 'btn btn-large btn-success floatright') do %> <%= content_tag :i, '', :class => 'icon-white icon-shopping-cart' %> Register and Checkout <% end %>
     <% end %>
</div>

and my attempt at reading the params (this is part of a large order controller create method)

if params[:is_register]
      Rails.logger.debug {"Registering before checking out"}
      @user = Member::User.new(params[:member_user])
      @user.address = Member::Address.new(params[:address])
      Rails.logger.debug {"address params " +params[:address].to_yaml }
      @user.pending_order = Checkout::Order.new()
      @user.pending_order.status = 'P'
      Rails.logger.debug {"Saving the new user record " + @user.to_yaml}
      Rails.logger.debug {"Saving the new user address record " + @user.address.to_yaml}
    end

    Rails.logger.debug {"Value of user " + @user.to_yaml}

    respond_to do |format|
      if params[:is_login] and user
        format.html { render :shipping_billing }
        format.json { render json: @order, status: :created, location: @order }
      elsif params[:is_register] and @user and @user.save and @user.address.save
        Rails.logger.debug {"Saved new customer account and address"}
        format.html { render :shipping_billing, :notice => "Your account was successfully registered"}
        format.json { render json: @order}
      else
        format.html { render action: "new", :notice => "Could not log in" }
        format.json { render json: @checkout_order.errors, status: :unprocessable_entity }
      end
    end

And finally the relevant model:

class Member::User < ActiveRecord::Base
  authenticates_with_sorcery!
  # attr_accessible :title, :body

  # Setup accessible (or protected) attributes for your model
  attr_accessible :email, :password, :password_confirmation, :address, :address_attributes, :first_name, :last_name, :order_attributes, :pending_order, :is_checkout

  attr_accessor :is_checkout


  # Orders and Order History
  has_many :orders, :class_name => "Checkout::Order"
  has_one  :pending_order,
           :class_name => "Checkout::Order",
           :conditions => ['status = ?', 'P']


  has_many :addresses, :class_name => "Member::Address"
  has_one  :primary_billing_address,
           :class_name => "Member::Address",
           :conditions => ['is_primary_billing = ?',true]
  has_one  :primary_shipping_address,
           :class_name => "Member::Address",
           :conditions => ['is_primary_shipping = ?',true]
  has_one :address,
          :class_name => "Member::Address",
          :conditions => ['is_primary_billing = ?', true]

  accepts_nested_attributes_for  :address
  accepts_nested_attributes_for  :pending_order

  validates_presence_of :email, :password, :first_name, :last_name
  validates_confirmation_of :password
  validates_uniqueness_of :email

end