I don't have AutoConfigBackup installed, never had but I got this warning after upgrading to lastest snapshot using
http://snapshots.pfsense.org/FreeBSD7/RELENG_1_2/pfSense-Full-Update-1.2.3-20090219-2152.tgz :
Feb 21 00:30:03 php: : New alert found: Either the username, password or encryption password is not set for Automatic Configuration Backup. Please correct this in Diagnostics -> AutoConfigBackup -> Settings.