class User < ActiveRecord::Base
    devise :database_authenticatable, :registerable,:recoverable, :rememberable, :trackable, :validatable
    validates_presence_of :first_name, :last_name
    has_many :subscriptions
    accepts_nested_attributes_for :subscriptions
    attr_accessible :first_name, :last_name, :email, :password, :password_confirmation, :remember_me, :subscriptions_attributes
    ...
end

class Subscription < ActiveRecord::Base
  belongs_to :user
  validates_presence_of :user_id, :chargify_subscription_id, :chargify_product_handle
  attr_accessible :user_id, :chargify_subscription_id, :chargify_product_handle
  ...
end

<%= form_for(resource, :as => resource_name, :url => registration_path(resource_name)) do |f| %>
    <%= devise_error_messages! %>
    <p><%= f.label :first_name %><br />
      <%= f.text_field :first_name %></p>
    ...

    <%= f.fields_for :subscription do |s| %>
        <p><%= s.label :chargify_subscription_id %><br />
          <%= s.text_field :chargify_subscription_id %></p>
            ...

Started POST "/users" for 127.0.0.1 at Sat May 14 12:38:49 -0700 2011
  Processing by Devise::RegistrationsController#create as HTML
  Parameters: {"commit"=>"Sign up", "authenticity_token"=>"wNZhZgIhYm9CpZfhvDiRBqaJseoO8QvR0Mk9VIybhcI=", "utf8"=>"✓", "user"=>{"password_confirmation"=>"[FILTERED]",        "last_name"=>"9", "subscription"=>{"chargify_product_handle"=>"medium", "chargify_subscription_id"=>"123"}, "password"=>"[FILTERED]", "first_name"=>"9", "email"=>"99@99.     com"}}
WARNING: Can't mass-assign protected attributes: subscription

<%= f.fields_for :subscriptions do |s| %>
    <p><%= s.label :chargify_subscription_id %><br />
      <%= s.text_field :chargify_subscription_id %></p>
        ...