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).
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.