Just to add a little PSA/followup, I have a Realtek RTL8111/8168B PCI Express Gigabit Ethernet controller (lspci said so), and there were a couple of options in the kernel menuconfig that related to my card. I don't remeber them specifically, but they were deep in the config. It was Device Support > Network Support > somethingicantrememberoffthetopofmyhead. So, yes, that was indeed the problem. Failure to know what hardware you use before installing is not cool.
A Public Service Announcement from your friendly neighborhood <strike>Spiderman</strike> UNIX user.