Microsoft has made RC1 (Release Candidate 1) of SP1 (Service Pack 1) available to the general public:
Microsoft Windows Server 2003 Service Pack 1 Release Candidate (32-bit)
Itanium-based versions of Microsoft Windows Server 2003 Service Pack 1 Release Candidate (64-bit)
NOTE: "Beta versions of software are intended only for installation in a test environment. We advise against installing and evaluating beta software on any production computers."
"Install Microsoft Windows Server 2003 Service Pack 1 (SP1) Release Candidate (RC) to help secure your server and to better defend against hackers. Windows Server 2003 SP1 RC enhances security infrastructure by providing new security tools such as Security Configuration Wizard, which helps secure your server for role-based operations, improves defense-in-depth with Data Execution Protection, and provides a safe and secure first-boot scenario with Post-setup Security Update Wizard. Windows Server 2003 SP1 RC assists IT professionals in securing their server infrastructure and provides enhanced manageability and control for Windows Server 2003 users."