Same Service packs folks
http://en.wikipedia.org/wiki/Small_business_server
"It must be noted that Windows Small Business Server is technically not an 'edition' of the Windows Server operating system but rather a customized SKU of server technologies targeted especially to small businesses. As such, the application servers are not merely bundled with the OS but are tightly integrated into the operating system. The same service packs as those for Windows Server or other server products cannot simply be used to update the OS."
It must be noted that this is no longer true. One can install Windows 2003 sp2 to SBS 2003. Since the R2 era (meaning post SBS 2003 sp1) the goal has been to not have special service packs for SBS, but rather be able to install the same exact service packs as Windows Server, Exchange Server, Sharepoint, WSUS, ISA and so on.
So let me restate that:
The SAME service packs as those for Windows Server or other server products CAN simply be used to update the Operating system.
SBS is just parts with extra magic.
Read the complete post at http://msmvps.com/blogs/bradley/archive/2008/03/29/same-service-packs-folks.aspx