Even though the service pack was officially released to the public this week, systems with certain configurations will find themselves blocked from automatic updates due to “compatibility concerns” between the update and their system. In particular, the “Microsoft Dynamics Retail Management System” has been targeted as the culprit, reportedly causing issues both with Windows XP SP3 and SP1 for Vista.
This is certainly not going to affect any home users anytime soon. As for retail outlets relying on Windows machines, they probably have an IT team in place to test these things first. There are a few known issues with SP3 regardless, but as it was just released today it will probably take some more time before they all come to light, and thus get a fix.