Multiple IPs on a single ethernet interface under OS X
Turns out it's easy, and you can even do it from the GUI. Just go to System Preferences:Network, select Show Network Port Configurations, and duplicate the interface you want two IPs on. Then edit the new interface's IP, et voila. That's for Tiger, anyway. Leopard uses the "+/-" style of list item duplication, but it seems to work more or less the same way.