I just check on Fedora 11, when run system-config-network, it prompt me with a dialog to enter root password from there I was able to make all the configurations I need. While you in system-config-network, and hit edit, disable Managed by network manager. I'll give you some pics as a reference, hold on.
I'm not using my eth0, I'm using wireless all the the time. I just installed the NetworkManager command line utility, cnetworkmanager and do my connections from there. For eth0, I do them in system-config-network. However, nm-applet should had allowed to manage you connections from there. Don't know why...