Vista has a worse uptake rate than ME did, so M$ has decided to eliminate a barrier. Those of you who did not want to face Vista's kill switch or nagware, will soon see your XP's WGA upgraded to Vista annoyance level.
WGA Notifications now looks and behaves more like WGA on Windows Vista in that it's naggier and annoying but still allows you to use the system even if flagged as non-genuine.
M$ has long used technical and contractural means to force the upgrade train, but I'm unaware of an "upgrade" that openly degrades an older version of Windows like this does. Most people are familiar with OEM CDs and EULA's that tied previous versions of Windows to a particular computer. They are also painfully aware of OS End Of Life issues like the end of security updates and application support that eventually dooms every version of Windows ever made. There have also been rumors of virus releases and SDK "updates" that intentionally break older versions. Making XP nag more takes sabotage to a whole new level.
M$ spokesmen swear that a kill switch will not be added to XP ever, but Vista as it was is where the company wants to take you. Their EULA has always granted them the ability to terminate your license to use their software at any time for any reason.
Discussion, is here, from Aristocat two days after this journal was published.