It only happens after the 10.5.8 update, and only (from what I can tell) on clients that were originally running Xsan 1.4, and were upgraded to Xsan 2.
serialnumberd, and I'm sure you're aware, is required for an Xsan client to work properly. With Xsan 1, this got launched by a Startup Item, namely /System/Library/StartupItems/SerialNumberSupport. With Xsan 2, this gets launched by a LaunchDaemon, /System/Library/LaunchDaemons/com.apple.SNServer.plist. If you happened to start with Xsan 1, and upgrade to 2 (and along the way probably went from Tiger to Leopard), you might have both. If you happen to have both, and you ran the 10.5.8 update, things might break.
It's easy enough to test. Just kill the Startup Item.
sudo SystemStarter stop SerialNumberSupport
Give it a minute. If your Xsan volumes suddenly decide to mount, then you should consider moving that Startup Item out of the way. It will bring you nothing but tears.