To display the network interface information for a particular network interface in Snow Leopard (not sure about other versions) use the following:

ipconfig getpacket interface

For example,
# ipconfig getpacket en0
htype = 1
flags = 0
hlen = 6
hops = 0
xid = 446704316
secs = 0
ciaddr =
yiaddr =
siaddr =
giaddr =
chaddr = 0:1e:c2:f:1b:45
sname =
file =
Options count is 8
dhcp_message_type (uint8): ACK 0x5
server_identifier (ip):
lease_time (uint32): 0x15180
subnet_mask (ip):
router (ip_mult): {}
domain_name_server (ip_mult): {,}
domain_name (string): domain.local
end (none):

This gives you much more information than a standard ifconfig command. The command above will display all the current options for the interface. You can narrow this down with the getoption parameter.