Login system across subdomains

Does anyone use the Zen Cart login system for other sites in their domain?

I have my shop set up in a subdomain, but would like to have the same login system across all subdomains and want to know if this something that can be done.

I have my shop set up in a subdomain, but would like to have the same login system across all subdomains and want to know if this something that can be done.
Look at this site.... http: //www.zen-cart.com/ is ZenCart
http: //www.zen-cart.com/forum is vBulletin
http: //www.zen-cart.com/tutorials/ is some knowledge base
http: //www.zen-cart.com/wiki/ is mediawiki

All of them have their own login system

Not saying it can not be done just that it is a lot harder than one would expect and then there are session downsides