When using FreeBSD version 4.5 with the SIS Onboard NIC, the MAC address is not recognized. It seems the sis driver needs an update patch. I've heard of similar problems with Intel onboard NICs and others.
sis0: <SiS 900 10/100BaseTX> port 0xd400-0xd4ff mem 0xeb800000-0xeb800fff irq 10 at device 1.1 on pci0 sis0: Ethernet address: 00:00:00:00:00:00
A simple solution is simply to set the MAC address yourself. In rc.conf add the "lladdr" portion to the "ifconfig" line. Get another MAC address off the net (usenet), or make one up. Just make sure it's unique on your local lan.
ifconfig_sis0="inet XXX.XXX.XXX.XXX netmask 255.255.255.0 lladdr 00:40:26:97:37:3A"
(Note: The above ifconfig syntax only works with versions of FreeBSD up to 4.5)
If you know of any problems this causes, I'd like to know.