So... if I choose live CD and type "ifconfig em0 up" followed by "dhclient em0" ... everything works... but if I go through the install, select something that isn't on the media and continue, selecting em0 as my network, and ipv4->DHCP ... I see "sendmeg on em0: No buffer space available" ...
If I don't choose anything off-media, but still choose ipv4->DHCP->em0 later, It fails inconsequentially, and on reboot it still says "sendmsg on em0: No buffer space available" ... but then it succeeds later after loading uhid.ko (just giving that for ordering... uhid.ko isn't loaded for the livecd, I don't believe).
The machine is and HP Compaq 8100 Elite with an i5 660 and 4G RAM.
The em0 is on the motherboard and probes as:
em0: <Intel(R) PRO/1000 Network Connection> port 0x3100-0x311f mem 0xf0400000-0xf041ffff,0xf0425000-0xf0425fff irq 19 at device 25.0 on pci0
em0: attach_pre capping queues at 1
em0: using 1024 tx descriptors and 1024 rx descriptors
em0: msix_init qsets capped at 1
em0: Unable to map MSIX table
em0: Using an MSI interrupt
em0: allocated for 1 tx_queues
em0: allocated for 1 rx_queues
em0: Ethernet address: 40:61:86:xx:xx:xx
em0: netmap queues/slots: TX 1/1024, RX 1/1024
em0: link state changed to UP
This link was also confirmed by firstname.lastname@example.org on the mailing list before I submitted this.