As I said a few days ago the Release Candidates for Windows XP and Vista are due to hit the public soon.. like today. The Super Site for Windows has updated their FAQs with new information you may wish to read before you decide to load these updates.
Windows Vista SP1 FAQ
Windows XP SP3 FAQ
Here's a link to this week's Windows Weekly from TWIT that goes over the Service Packs and more..
And here's a tool from Microsoft that will let you keep from loading the SPs until they go live later on in 2008.