carmen-rails is a Rails 3 plugin that supplies two new form helper methods: country_select and subregion_select. It uses carmen as its source of geographic data.

Just add carmen-rails to your Gemfile:

gem 'carmen-rails', '1.0.0.pre'

<%= form_for(@order) do |f| %>
  <div class="field">
    <%= f.label :country_code %><br />
    <%= f.country_select :country_code, priority: %w(US CA), prompt: 'Please select a country' %>
  </div>
<% end %>

More docs coming soon.

There is a live demo app that shows carmen-rails in action.