I tried to configure postfix to use email forwarding, both by postfix configuration and by the use of a .forward file. It failed silently. Eventually I found out that it was the fault of SELinux, which was enabled during my installation of Fedora Core 5. There doesn't seem to be any fix I know off other than turning it off via the command: setenforce 0.
With selinux disabled, procmail does invoke my whitelist filter correctly.