Yeah, my nic supports 10/100, and so does my router. All of the other computers on the network run at 100.
ifconfig:
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
inet 127.0.0.1 netmask 0xff000000
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
stf0: flags=0<> mtu 1280
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet6 fe80::200:94ff:fea1:4d95%en1 prefixlen 64 scopeid 0x4
inet 192.168.1.104 netmask 0xffffff00 broadcast 192.168.1.255
ether 00:00:94:a1:4d:95
media: autoselect (10baseT/UTP) status: active
supported media: 10baseT/UTP 100baseTX 10baseT/UTP <full-duplex> 100baseTX <full-duplex> autoselect autoselect 10baseT/UTP
en0: flags=8822<BROADCAST,SMART,SIMPLEX,MULTICAST> mtu 1500
tunnel inet -->
ether 00:03:02:ac:19:42
media: autoselect (<unknown type>)
supported media: none autoselect 10baseT/UTP <half-duplex> 10baseT/UTP <full-duplex> 100baseTX <half-duplex> 100baseTX <full-duplex>