Almost entirely painless!
OK, so not much more than an hour later, and my upgrade from RC2 of Windows Server 2003, Web Edition to the RTM version is complete, and seems to have gone almost entirely smoothly. I say almost, because despite the fact that I was upgrading rather than doing a fresh install, I still had to manually install the VMWare SVGA video driver. I also had a sudden shutdown of the VM, but I'm not sure yet whether that was just a VMWare hiccup, or something more ominous.
But it's up and running right now, and I'm installing the sample files from my latest book, Microsoft® ASP.NET Programming with Microsoft Visual Basic® .NET Version 2003 Step By Step, to make sure they work OK on the RTM release. Will report back.
Meanwhile, one minor gripe about VMWare. I desperately want them to start supporting the third and fourth mouse buttons on Microsoft's (and others') optical mice. I live by these buttons for forward and back in both IE and Windows Explorer, and it's really irritating that I can't use them in a VMWare VM.