When upgrading a Mac from Mac OS X 10.4 (Tiger) to 10.5 (Leopard), remember that network accounts are not included if you do an archive and install and choose to migrate existing users. If a network account had its home folder at /Users/jbloggs then it will have been moved to /Previous Systems.localized/2009-11-06_0346/Users/jbloggs (although the date portion will be the date that you did your install).
This applies to network accounts which authenticate against Active Directory and do not have a mobile account.
Why my place of work used to setup Macs with the option for create mobile account at login turned off is a mystery to me.
