Ok, bottom line at this point of the game is don't do it. There seem to be plenty of things that Windows Server SP2 could break ( including network connectivity, backups, SBS console and more ). There also seems to be issues with uninstalling the sucker. Since there doesn't appear to be any real burning need to install this service pack on your SBS server I'd just hang back and let someone else bleed to death.
Even though it appears quite a few (unknowing) folks have installed SP2 on their machines ( thanks mainly to automatic updates and WSUS ) there doesn't really seem to be any concrete information about what or how the best way to install (or uninstall) the service pack or what needs to be upgraded before hand. It appears that things like HP broadcomm network cards needs to be upgraded but even that appears to be vague.
The hardest issues to resolve are related to certain chipsets of network cards, typically the Broadcom chipset. Here's an overview of this and some other problems:
• VPN, network and connectivity issues. You may find issues with your ISA 2004 firewall, Outlook connectivity, Remote Desktop, or VPN (Virtual Private Network). If you have Broadcom NICs (network interface cards), first make sure you have the latest drivers installed. Then, use the Registry to disable RSS (Receive Side Scaling) and Task Offloading, as discussed in the SBS blog and the ISA Server blog.
• Help and Support Service missing. Seemingly the largest issue, and one that's easily fixed, is that the Help and Support Service appears to be missing. The instructions to reinstall this service are discussed on the SBS blog for that platform.
• Issues caused by uninstallation of SP2. Issues caused by removing SP2 include scheduled tasks being altered so you must reenter your passwords. Also, unless you have Windows 2003 R2 or SBS 2003 R2, removing SP2 causes the rollback of MMC 2.0 to have issues. To resolve this, review the known issues in the release notes and remove the files from the %APPDATA%\Microsoft\MMC\ folder.
For the time being, hold off installing Windows Server SP2 on your SBS 2003 servers.