Microsoft Knowledge Base Article 897342 contains the following summary and introduction:
SummaryThis article describes the top known issues that you may experience if you install Microsoft Windows Server 2003 Service Pack 1 (SP1) on a computer that is running Microsoft Windows Small Business Server 2003.
INTRODUCTIONWindows Server 2003 SP1 is supported on Windows Small Business Server 2003. However, there are known integration issues that will be resolved in Windows Small Business Server 2003 SP1.
Although Windows Server 2003 SP1 is now available for public distribution, we recommend that you wait until Service Pack 1 for Windows Small Business Server 2003 is released before you install this service pack.
If you have already installed Windows Server 2003 SP1, you do not have to remove the service pack because it is a prerequisite to installing Windows Small Business Server 2003 SP1. However, if you must remove Windows Server 2003 SP1 to address a known issue, you will be required to reinstall the service pack when you install Windows Small Business Server 2003 SP1 after it becomes available.