The upcoming service pack for Windows XP has been covered by many. Until actual official releases are made, though, it's really hard to say how it will turn out. Now, if you are a MSDN member or a TechNet subscriber, you can get access to the the Release Candidate 1 of the service pack.
It isn't a public RC, and of course Microsoft will frown upon you should you choose to distribute it, but if you do happen to have access you can download it right now. This will definitely be a boon to admins who want to begin testing the service pack or for those who just want to see how it behaves.
Both XP's SP3 and Vista's SP1 have been covered in depth the past few months, as both of them are approaching their release dates.