Does uPortal support single-sign in?

Yes. The uPortal framework includes an authentication system that explicitly supports the notion of single sign-on. The authentication provider architecture is highly pluggable and can be outfitted with a provider appropriate to your chosen SSO architecture. If your SSO architecture has a mechanism to set the REMOTE_USER header from the perspective of the portal (many Apache authentication modules adopt this approach), then you may be able to use the RemoteUserSecurityContext included with uPortal.

uPortal further supports the use of proxiable credentials, a feature of advanced single sign on frameworks such as JA-SIG's very own Central Authentication Service. Several (but not all) of the channels that ship with the framework can take advantage of this to securely access backing sources of user-specific information.

The authentication and LocalConnectionContext APIs are highly pluggable. Any campus that wants to write their own channels that will use the uPortal single sign-on capabilities are certainly able to do so.