The nmap security checking tool appears to have some trouble detecting the network adapters on Leopard (Mac OS X 10.5). To make it work one must add the following command-line parameter (to use the built-in NIC):

-e en0

One can discover the other network interfaces with

sudo ifconfig

Ordinarily en0 is the built-in ethernet port and en1 is the wireless adapter.

zenmap

If you use the handy zenmap user interface for nmap, you can do several things. For example, you can edit the profiles, or just include this in the target field (-e en0 target machine name). I edit the profiles that I use and set the network interface. Go to the Source tab, check Set network interface (-e), and type en0 in the edit field.

2008-12-04 Update:
* Added more details for zenmap, hyperlinks.