sitemap.xml.erb

<% base_url = "http://youdomain.com/" %>
<% pages = sitemap.resources.find_all{|p| p.source_file.match(/\.html/) } %>
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<% pages.each do |p| %>
  <url>
    <loc><%= base_url + p.destination_path.gsub('index.html','') %></loc>
    <% if priority = p.metadata[:page]['priority'] %>
      <priority><%= priority %> </priority>
    <% end %>
    <lastmod><%= File.new(p.source_file).mtime.to_date %></lastmod>
  </url>
<% end %>
</urlset>

Don't forget to add

page "/sitemap.xml", :layout => false

to your config.rb