If you slipstream SP1 into Windows Server 2003 then you'll need to add an extra file to the root of your CD! With all other service pack it adds a Service Pack Identifier file to the root of your CD directory, but SP1 for Windows Server 2003 currently does not do this, so I have provided the 3 files for you to manually install in the root of your CD (not in the I386 directory)! Just download them into the Root of your CD, eg c:\Win2K3\ and it will now work! If you do not have this file in the root of the CD then Windows Setup will ask you to provide the SP1 setup disks during the install!
Guide:
http://www.betaplace.co.uk/ssp1.asp