This laptop has two network interfaces. eth0 is built-in, and eth1 is a removable CardBus adapter. My /etc/sysconfig/network-scripts/ifcfg-eth1 has ONBOOT=no, but if I have my CardBus card in when I boot, it is still enabled and configured.
Created attachment 94805 [details] config file for eth1
It's brought up by hotplug, which doesn't really have a concept of 'boot' or not.
HOTPLUG=no works for me