Microsoft has finally released Service Pack 3 for Windows XP.  There are different versions of it available using the URLs below:

Windows XP Service Pack 3 Network Installation Package — 316.4MB
http://www.microsoft.com/downloads/details.aspx?FamilyID=5b33b5a8-5e76-401f-be08-1e1555d4f3d4&DisplayLang=en

Windows XP Service Pack 3 ISO File — 544.9MB
http://www.microsoft.com/downloads/details.aspx?FamilyID=2fcde6ce-b5fb-4488-8c50-fe22559d164e&DisplayLang=en

The ISO file seems to be similar to be similar to the version Microsoft put together when they released Windows XP Service Pack 2 on CD for free.

You do have the option to get a smaller version of Service Pack 3 if you use Windows Update.  Go to the URL below for this version and follow the download and installation directions.  This download size will vary depending on how many updates have already been installed on your system.  I did this kind of installation on a Windows XP Service Pack 2 in VMware Fusion, that already had very update possible, and the download was 66.9MB.
http://windowsupdate.microsoft.com

Release notes for Windows XP Service Pack 3 are available at the URL below.
http://download.microsoft.com/download/c/d/8/cd8cc719-7d5a-40d3-a802-e4057aa8c631/relnotes.htm

One point that Microsoft makes in the release notes is that if you have Internet Explorer 7 or the beta Internet Explorer 8 installed BEFORE you install Service Pack 3, you will not be able to uninstall it without first uninstalling Service Pack 3.  An explanation of this situation can be found at the URL below.
http://blogs.msdn.com/ie/archive/2008/05/05/ie-and-xpsp3.aspx

Happy downloading and installing.  Post your experience as a comment for the rest of the community.