Небольшой яваскрипт класс, позволяющий устанавливать и получать значения cookies для поддоменов сайта.

var Cookie =
{
   set: function(name, value, days)
   {
      var domain, domainParts, date, expires, host;

      if (days)
      {
         date = new Date();
         date.setTime(date.getTime()+(days*24*60*60*1000));
         expires = "; expires="+date.toGMTString();
      }
      else
      {
         expires = "";
      }

      host = location.host;
      if (host.split('.').length === 1)
      {
         // no "." in a domain - it's localhost or something similar
         document.cookie = name+"="+value+expires+"; path=/";
      }
      else
      {
         // Remember the cookie on all subdomains.
          //
         // Start with trying to set cookie to the top domain.
         // (example: if user is on foo.com, try to set
         //  cookie to domain ".com")
         //
         // If the cookie will not be set, it means ".com"
         // is a top level domain and we need to
         // set the cookie to ".foo.com"
         domainParts = host.split('.');
         domainParts.shift();
         domain = '.'+domainParts.join('.');

         document.cookie = name+"="+value+expires+"; path=/; domain="+domain;

         // check if cookie was successfuly set to the given domain
         // (otherwise it was a Top-Level Domain)
         if (Cookie.get(name) == null || Cookie.get(name) != value)
         {
            // append "." to current domain
            domain = '.'+host;
            document.cookie = name+"="+value+expires+"; path=/; domain="+domain;
         }
      }
   },

   get: function(name)
   {
      var nameEQ = name + "=";
      var ca = document.cookie.split(';');
      for (var i=0; i < ca.length; i++)
      {
         var c = ca[i];
         while (c.charAt(0)==' ')
         {
            c = c.substring(1,c.length);
         }

         if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
      }
      return null;
   },

   erase: function(name)
   {
      Cookie.set(name, '', -1);
   }
};

Использовать:

Cookie.set('mycook', '123'); // установили куки
Cookie.get('mycook'); // получили куки
Cookie.erase('mycook'); //удалили куки

источник http://developers.livechatinc.com/blog/setting-cookies-to-subdomains-in-javascript/