Followup to session question / scope of session

If I start a session at www.mydomain.com and want to register a session variable to keep track of the user for authentication purposes, is there a way I can use that session variable on other sites, such as my.mydomain.com and mydomain.com or anythingelse.mydomain.com? It seems to only be valid for the www. domain name, and I need it to work on all of *.mydomain.com. I could set a cookie with a value, but I don't want to risk spoofing of cookies, etc.. I'd much rather keep a registered session variable which I can check to see if it exists, and use that to authenticate. Thanks, Chad