I just finished installing and configuring SP1 for SBS 2003. I ordered the cd-rom about a week ago and received it on Saturday. Today looked like a good day to install it. It took me between 4 to 6 hours to install and configure it. I had a little extra work because I have the Premium version which means I need to upgrade SQL Server and ISA 2004. Since I had previously migrated Sharepoint from MSDE to SQL Server, I had to install SQL SP4 twice, once for default instance and once for sharepoint instance.
I found the advice in the article How to install Service Pack 1 for SBS 2003 [Author Mariëtte Knap] to be helpful. I had to disable IISAdmin to complete the installation. Then I had to re-enable IISAdmin and make sure the IIS server was started to complete the Internet Connection Wizard without errors. ISA 2004 is pretty slick!
I also had to re-install the firewall client on my workstation. I was somewhat surprised I had to alter my configuration in Firefox to get it to work again. Oh well! I guess I have some more testing to do.