The first steps of adding in new functionality to your server is available. Download here
New functionality added that are of interest are:
Stronger defaults and privilege reduction on services
Support for “no execute” hardware
Network Access Quarantine Control components included - Interesting information on this functionality here
IIS 6.0 metabase auditing - Nice feature especially if it the metabase becomes corrupt. You can track down who is responsible for the corruption.
New Features to Server 2003
Windows Firewall - The change here allows for the firewall to provide security at boot up time where in past when the network stack loaded and when Internet Connection Firewall started there was time potentially to inject packets to the server. As well unattend installation is now available where it was not in the past.
Post-Setup Security Updates (PSSU) - Probably the best feature added to this update. It blocks incoming traffic to the server until it has been updated and then it disengages the Firewall to allow the ports to be configured depending on the server type decided.
Security Configuration Wizard (SCW) - Wizard that configures server security based upon existing server roles. SCW asks questions about server roles and then stops all services not necessary to perform those roles. This Feature comes in a close second in my opinion.