On two separate servers I tried to apply SP3 with RU1 integrated. One was a 64 bit Windows 2008 SP2 machine with Exchange 2007 SP2 w/ RU4. It has the CAS/HTS/MBX roles installed.
The other was a 32 bit Windows 2003 SP2 machine with Exchange 2007 SP2 with RU4. It just had the Admin console as it was running DPM 2007 SP1.
On both machines the error of "Installing product excahgneserver.msi failed. Fatal Error during installation. Error code is 1603.
On the actual Exchange server, fixing it required removing the RU1 from the Updates folder of SP3, manually copying over the CHM file (since most Exchange files were removed as a part of the upgrade), restarting web services, and re-running SP3. Once SP3 succesfully applied, and I got a change of underwear, I applied RU1 separately.
I did disable Forefront protection before attempting SP3 w/ RU1 as indicated in this post.
The DPM server required copying the CHM file over as it didn't really have IIS integration.
With the one server I was willing to chalk it up to something wacky about the machine. With two separate servers acting the same way w/ 2 different versions (32bit versus 64bit and Windows 2008 versus Windows 2003) I think it's the code not liking applying the RU1 in conjunction with SP3.
Has anyone run into this?