RedmonMag.com has a good article on Windows Server Core which will be a version of Longhorn server stripped down to the bare essentials for certain infrastructure roles such as domain controller and file/print servers. Server Core has no GUI, a much reduced set of services and applications, etc. The purpose is to reduce the attack surface and footprint of the base OS. This is the beginning of a longer journey to continue to modularize the system.
For those of you with large branch office infrastructures, the combination of Longhorn Server Core and Longhorn's Read Only Domain Controller capability provide for an extremely locked down configuration.