Ok, so I wasn’t sure about this, but tried. it.

Create a batch file with this in it and execute:

@echo off

reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\VistaSp1 /f > NUL 2>&1
reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\WindowsUpdate\VistaSP1 /f > NUL 2>&1

reg add HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\VistaSp1 /v Beta1 /t REG_SZ /d dcf99ef8-d784-414e-b411-81a910d2761d /f
IF NOT %errorlevel% == 0 ( goto ERROR)

:SUCCESS
@echo.
echo ===========================================================
echo  Windows Vista SP1 registry key has been set successfully.
echo  Please check for updates in Windows Update.
echo ===========================================================
@echo.
goto END

:ERROR
@echo.
echo ===========================================================
echo  FAILED to set Windows Vista SP1 registry keys.
echo  Please run this script by right clicking and selecting
echo  "Run as Administrator".
echo ===========================================================
@echo.
goto END

:END
pause

Then run windows update. I had to install and reboot twice with two updates first before I got the SP1 download. In Windows Update it says it is the RC1 version, but after install my build is 6001.18000.x86fre.longhorn_rtm.080118-1840

which I have read in various forums is the RTM version that matches Windows Server 2008 RTM

Works for me. If it isn’t RTM, I will just update when it comes out.