Microsoft made available on December 3 for download the near-final Release Candidate (RC) of Windows XP Service Pack (SP) 3. The RC is available to all Microsoft Developer Network (MSDN) and TechNet subscribers. This isn't yet a generally available, "public" RC of XP SP3; that is still coming at some future date that Microsoft hasn't disclosed. Microsoft provided an RC "preview" of XP SP3 to 15,000 testers in mid-November.
Microsoft is still on track to ship the final XP SP3 in the first half of calendar 2008. XP SP3 is a rollup of all previously released XP updates, according to Microsoft, "including security updates, out-of-band releases, and hotfixes."
Lately, Microsoft has been releasing XP SP3 and Vista SP1 builds in lockstep. (The same 15,000 testers that got XP SP3 RC preview also got the Vista SP1 RC preview.) Given that Microsoft told its own employees to expect the company to make the RC of Vista SP1 more broadly available some time this week, it seems logical that the Vista SP1 RC is right around the corner, too.
News source: Mary Jo Foleys Blog