In addition to roles and features that are included with Windows Server 2008 by default, Server Manager enables integration of additional roles and features that are available on the Microsoft Download Center and Windows Update Web sites as optional updates to Windows Server 2008.
One role that will be available as an update is Windows Server Update Services 3.0 Service Pack 1 (WSUS 3.0 SP1). This update to Server Manager enables full integration of WSUS 3.0 SP1 into Server Manager, allowing installation, configuration, and management of WSUS 3.0 SP1 using the Server Manager console and wizards.
Beta testers can download a server manager update for Windows Server 2008 RC0 to enable this new optional role, for now that is just WSUS. But since MS removed Windows Sharepoint Services as default available rol on WS08 I think this will become an optional role for Server manager too.