This has been a long-running battle I've had with Microsoft over the years - the inability to either roll my own slipstreamed SBS media, or obtain slipstreamed SBS media at reasonable cost (like the Volume License media kits).
Why would I want to roll my own or access low-cost SBS media, you ask?
Well, if you ever have to do a Repair Install of your SBS box and you've added a Service Pack, you're hosed. The Repair Install will undo the service pack applied to the once-working box, and on reboot will give you a lovely blue screen. The only way around this is to restore from the last full backup, or re-image from your favorite imaging tool of choice (two listed below if you're not already using one).
Thanks to the wonderful advances of products like Acronis True Image and ShadowProtect, performing Repair Installs are a thing of the past, especially if you're using Repair Install to migrate a Retail SBS install to new hardware. The hardware independent restore capabilities of these products makes a full backup / minimal install / full restore / repair install redundant, as well as a lot slower.
But it's a pain if you don't have these, so make sure you have SBS media from the same channel (OEM, Retail or Volume License) with the Service Pack you're running already slipstreamed. Or simply don't install Windows Server 2003 Service Packs to SBS 2003 boxes.