Apparently Microsoft has much less interest in service packs for XP than they do in releasing Vista. While that is understandable, it seems to indicate they are not terribly concerned with problems that exist in XP now.
Here is Microsoft's roadmap for service packs.
I frequently get the idea that Microsoft simply does not have enough people to deal with all the updates and upgrades they need to be putting out. With all their billions, one would think they could and would hire enough good people to do what they and their customers need done.