Microsoft first introduced Service Pack 2 for Windows XP back in 2004. There was initially a lot of gnashing of teeth and rending of garments over it because some of the new features in XP SP2, such as the firewall, broke many custom and some commercial applications. Many organizations chose to sit out Service Pack 2 entirely, and just stuck with SP1 on their workstations. That strategy was fine so long as Microsoft was going to continue supporting Service Pack 1. However, just as it stopped supporting Windows 9x over the summer, Microsoft XP Service Pack 1 is relegated to History.
With Vista shipping at some point in the near future, I'm sure that throwing the old software over the side makes support issues a lot easier for them. It's hard enough properly supporting one operating system, not to mention one that's several years or several revisions old. Because many people still use Windows 98, I was more irritated by its abandonment than I am about XP SP1.
Windows 2000 Professional will still be supported until the end of the decade and Windows XP itself probably won't be fully abandoned any time in the forseeable future. That date won't be determined until Windows Vista finally ships. Windows XP SP1 was more of a stop gap solution until organizations got used to what was new in SP2. As such, and two years later, it makes little sense to keep supporting it.
Microsoft has quietly been abandoning it for some time. Most of the new features that you can download for XP, such as the Desktop Search feature, require XP SP2. All this formal abandonment does is make official what's been happening slowly for over a year.
So: Farewell Windows XP SP1. Now we just have to wait until Microsoft gets around to shipping Service Pack 3.