KB 938759 - Resolves an issue where you cannot distribute or install a software package in Windows Server 2003 if the software package contains a very large signed CAB/EXE file - like Vista SP1

If you want your Windows 2003 WSUS servers to be able to download and deploy Vista SP1 when it comes out, you will want to install this update.

I suspect part of the delay with the release of Vista SP1 RTM is due to this issue with Windows 2003, but also the known driver reinstall problem.