Persisting authentication info when browsing to a subdomain

[Greg]

Is it possible for authentication using asp.net membership to persist
from a
site to a subdomain?

The scenario I want to provide is:
User browses to the page somesite.com and logs in.
User clicks on link that points to the subdomain demo/somesite.com.
Is it possible to retain the authentication information for the
subdomain, so that there is no need to log in again?

To clarify - what I am trying to do is to have a login page on my main
site. Once the user is logged in, they can browse to a subdomain, for
which all the pages will be restricted based on role. If the user is
logged in with the allowed roles, they can browse all pages within the
subdomain.

Regards,

Greg