Based on the work of Sam Beam and Bastian Schnurle.
Include this code in your view:
<% form_for :user, :url => :subscriptions do |f| %>
<%= text_field_tag :email, t('enter_your_email'), :class => 'email' %>
<%= submit_tag t('subscribe') %>
<% end %>
Copyright (c) 2012 Martijn Lafeber, released under the New BSD License