Apple has posted a kbase on using Xsan with 10.4.6, namely, that a global .plist file must be updated for everything to work soundly.

This fix needs to be performed on any node (client or controller) that has been updated to 10.4.6. On the kbase page, there is a disk image with a simple .plist file you use to replace the currrent /System/Library/Filesystems/acfs.fs/Contents/info.plist.

If one of your users has mistakenly (or rebelliously) updated to 10.4.6, check out the article to see how to get things stable again.

http://docs.info.apple.com/article.html?artnum=303549

Cheers,
Matt