When attempting to create a new Virtual Network, virt-manager 0.4.0 only allows networks to be created using private IP address ranges. Public IP ranges should be allowed (perhaps with a warning/confirmation), especially when creating isolated virtual networks.
Change to allow public addresses (with a confirmation dialog) is now upstream. http://hg.et.redhat.com/virt/applications/virt-manager--devel?cs=af3dcc2d8e57
virt-manager-0.5.3-1.fc7, gtk-vnc-0.3.2-1.fc7, python-virtinst-0.300.2-1.fc7, virt-viewer-0.0.2-1.fc7 has been pushed to the Fedora 7 testing repository. If problems still persist, please make note of it in this bug report. If you want to test the update, you can install it with su -c 'yum --enablerepo=updates-testing update virt-manager gtk-vnc python-virtinst virt-viewer'
virt-manager-0.5.3-1.fc7, gtk-vnc-0.3.2-1.fc7, python-virtinst-0.300.2-1.fc7, virt-viewer-0.0.2-1.fc7 has been pushed to the Fedora 7 stable repository. If problems still persist, please make note of it in this bug report.