Well it seems as though its finally happened-Service Pack 3 for every-ones favourite OS, Windows XP has been released to manufacturing (however not available to the public just yet-expect to see it on April 29th).

Service Pack 3 updates all 32-bit versions of Windows XP from Starter to XP Professional (the x64 edition of XP is based on Server 2003 and requires the Service Packs for that product). The complete package from the Download Center will reportedly be some 320 MB. Downloads via the Update function will be around 70 MB according to Microsoft’s current plans; this update can be so much smaller because only the data required for a specific XP version are downloaded, not the entire package.

Support for Windows XP without any service packs expired long ago and officially SP2 has to already be installed before SP3 can be installed, despite the fact there is no technical reason for this requirement. However Microsoft is inconsistent and SP3 can in practice be installed on XP with only SP1. Strangely, the complete SP3 contains all of the patches you need to update even a fresh base version of XP. Microsoft says that a slipstream installation CD can be created so that the operating system with SP3 can be installed at once without any other service pack.

SP3 not only contains patches and updates, but also a number of add-onsthat have been sold separately, such as Background Intelligent Transfer Service (Bits) 2.5, Windows Installer 3.1, Management Console (MMC) 3.0 and Core XML Services 6.0. SP3 does not, however, contain any fundamentally new functions, and no new versions of Internet Explorer or Media Player are included.

This is set to be the very last Service pack for XP however patches and updates for the OS are set to continue until Service Pack 3 expires in 2014.
A time line of SP3:

An overview for SP3 is available here (MSFT), however expect new documentation to arise pretty soon.