Just getting back to this, due to some testing with Ubuntu 13.04. Digging through the errors shown in the ALT+F4 window, I finally found a launchpad bug that pointed me towards this option (formatted for use in cobbler's sample.seed):

d-i live-installer/net-image string http://$http_server/cobbler/links/$distro_name/install/filesystem.squashfs

Using this, I am able to netboot 12.10 and 13.04 without hitting that screen that says the "Install the System" step failed. Hope that helps you guys out.