Sadly, some of us have to test these things before they go public. At work, WSUS will hold it back until I approve it for deployment. Whether I choose to let it loose to one machine, or 700, depends on how well it passes testing.
For those of you in a similar situation, modifying the registry with the following command will enable download of XP SP3 from Microsoft Update
reg add HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\XPSP3 /v RCPreview /t REG_SZ /d 1c667073-b87f-4f52-a479-98c85711d869 /f
If you wish to reinstate your machine to a previous state of update eligibility, use the following registry command
reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\XPSP3 /f
Those of you running Vista SP1 will already be aware of a similar hack, and it shows an increasing trend by Microsoft of using their update mechanism to deliver betas and RCs ahead of a full deployment. In the long run, it makes it nice and easy to prep a machine for testing.