I have a website here at http://harrisonprograms.com. You can register, login, logout all that good stuff but when you go to a subdomain such as blogs.harrisonprograms.com the login session doesn't take place on the subdomain.
It uses PHP sessions. So what I'm saying is all domains and subdomains can sign in but don't carry the session to a different domain. For example harrisonprograms.com won't carry the session over to blogs.harrisonprograms.com.