Microsoft is doing a "phased rollout" of XP SP2, both so corporate IT departments can get up to speed with it (the security enhancements have a good chance of breaking some things), and to keep their download servers from getting overwhelmed (the full version of XP SP2 is over 250 MB).
MSDN subscribers (like me; typically people who write Windows software for a living) had the first crack at it, as of last Friday. Then people who had installed pre-release versions of XP SP2 (as of yesterday) got to see it on Windows Update. But the general-release Windows Update version won't be out for a few more weeks. Note that if you don't have broadband, you should wait for the Windows Update version, rather than trying to download the MSDN/corporate 'full version'; the Windows Update download is much, much smaller as its customized for your PC.