Not sure if this has been reported before....
In OS X it is possible to assign more than one IP address to a single ethernet card. You might use this for instance if you wanted to use one IP as an external (public) IP and another as an internal-only secure IP for admin or whatever your needs are.
I was trying to do this via the Terminal in the same way it is done in other Unix variants. That method is:
% ifconfig eth0:2 192.168.1.132 upHowever, it's even easier in OS X. I cannot take credit for discovering this though - it is listed in an article at AFP548.
Let's say you currently have your machine IP set to 192.168.1.100 and you want to add a virtual IP address of 192.168.1.101 so both IP's point to your machine.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020919053409391