I don't usually post about technical stuff here, but Apple doesn't have this on their site and I just figured it out on my own, so I wanted to share.
If you have XSAN 1.1 installed on OS X Server and you uninstall it by running the "Uninstall XSAN.pkg" that came on the CD, after the required restart Server Admin will report in red that you have an "Invalid Serial Number". If you look at the Settings tab, under your completely valid serial number it will note "Invalid serial number: could not check license with daemon."
Apple acknowledges the problem, but their only solution is to call Apple Care. When I did so, the Server tech I talked to had no idea of any solution and said she'd escalate it to Engineering and they'd get back to me in 3 - 4 days.
So here's what you do: Reinstall XSAN 1.1. This will restore the system components that are needed to validate your serial number. Download the XSAN 1.2 Uninstaller. (I actually updated to XSAN 1.2, thinking that would give me the 1.2 Uninstaller, which it didn't, but I think the 1.2 Uninstaller will uninstall 1.1 also -- but I don't want to reinstall everything to check.)
Ta Da! XSAN uninstalled, valid serial number, no three day wait. Why that's too complicated for Apple to document, I'm not sure.