Capfile

server 'localhost', :test
server '127.0.0.1', :test2

task :test_serially do
  serially do
    run 'date && sleep 5'
  end
end

def serially(&block)
  original = ENV['HOSTS']
  find_servers_for_task(self.current_task).each do |server|
    ENV['HOSTS'] = server.host
    yield
  end
ensure
  ENV['HOSTS'] = original
end

output.txt

$ cap test_serially
  * executing `test_serially'
  * executing "date && sleep 5"
    servers: ["localhost"]
    [localhost] executing command
 ** [out :: localhost] Sun Aug 22 16:38:45 EDT 2010
    command finished
  * executing "date && sleep 5"
    servers: ["127.0.0.1"]
    [127.0.0.1] executing command
 ** [out :: 127.0.0.1] Sun Aug 22 16:38:51 EDT 2010
    command finished