There is speculation and rumors that with the Vista Service Pack 1 release, that Microsoft is intentionally delaying the release of Service Pack 3 for Windows XP. The testers in the Windows XP SP3 community all expected that it would be released this week, however this is not so, instead Microsoft released a beta version of SP3.
The ideas that are floating around are basically put: Vista SP1 is getting all the marketing hoopla whereas XP SP3 is delayed for no reason. Essentially Microsoft is be portrayed as not caring about support for XP and promoting Vista at its detriment. I do not know what the reality is, however as it goes for most things, its probably somewhere in the middle. I'm sure Microsoft is not happy with the resistance it has faced with the Vista upgrade, but at the same time I doubt they are willing to alienate their existing customers.