Blog: Extract the Google Analytics Visitor ID from the cookie and push it back to GA as a custom variable.

ga_tracking.js

// Extracts the Visitor ID from a GA cookie
function _goog_id(){
  var id, a, c = document.cookie.split('; ');
  for(i in c){a = c[i].split('=');if(a[0]=='__utma'){id = a[1].split('.')[1];}}
  return id || 'unknown';
}

var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-XXXXX-X']);
_gaq.push(['_trackPageview']);
_gaq.push(function(){_gaq.push(['_setCustomVar',5,'VisitorID',_goog_id(),1]);});

(function() {
  var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
  ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
  var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
})();

neat code, I have updated my blog post with reference to your version (much cleaner than my version)