If you are running Small Business Server then don't apply this Service Pack! Yes I know, you think that if it applies to Windows 2003 Server then why not to SBS 2003. Take it with philosophy, before SP1 is available to SBS many others would have already tested it before you. Read more from here:
Coming Soon: Windows Small Business Server 2003 Service Pack 1

We added this to our main SP1 issue list article. Check this (see references) for the complete list.

References

Windows 2003 SP1 Exchange Issue List