This script gets the URL of your frontmost Safari tab, finds the YouTube MP4 URL for it and passes it to your AppleTV. You need to install the rb-appscript and airplay gems first. Tested on OS X 10.9 only.

mytube.rb

#!/usr/bin/env ruby

##
## This script gets the URL of your frontmost Safari tab, finds the
## YouTube MP4 URL for it and passes it to your AppleTV.
##

require 'airplay/cli'
require 'appscript'
require 'net/http'

def get_safari_url()
  url = Appscript::app('Safari').documents[1].URL.get
  
  if not /youtube\.com/.match(url) then
    puts 'Your frontmost Safari tab does not contain a YouTube video.'
    exit 1
  end

  return url
end

def get_youtube_mp4(yt_url)
  yt_uid = yt_url.split('=').last
  yt_url = 'http://www.youtube.com/get_video_info?video_id=' + yt_uid

  res = Net::HTTP.get_response(URI(yt_url))
  map = res.body.split('url_encoded_fmt_stream_map=').last
  map = URI.decode(map.split('&').first)

  map.split(',').each { |fragment|
    type = ''
    video_url = ''

    fragment.split('&').each { |parameter|
      if /^sig=/.match(parameter) then
        video_url = video_url + '&signature=' + parameter.split('=').last
      elsif /^url/.match(parameter) then
        url = parameter.split('=').last
        video_url = URI.decode(url) + video_url
      elsif /^type/.match(parameter) then
        type = URI.decode(parameter.split('=').last)
      end
    }

    if /^video\/mp4/.match(type) then
      return video_url
    end
  }
end


begin
  video_url = get_youtube_mp4(get_safari_url())
  
  Airplay::CLI.play(video_url,
                    { device: Airplay.devices.first })
rescue Interrupt
end