Note: Time Machine has default exclusion list that prevents it from backing up some important files in OSX Server. (Ref http://www.bill.eccles.net/bills_words/2008/08/designed-to-fail-apple-time-ma.html).

The file to edit to modify the exclusion list is as follows.

/System/Library/CoreServices/backupd.bundle/Contents/Resources/StdExclusions.plist

Update: Checked up on this in 10.6.3 and it seems that /var/spoool is now backed up.