I’ve been using a nice little piece of software called Marco Polo. It’s purpose is to detect which network you are connected to and automatically switch you to it. It can do this according to the IP address that you are assigned or by detecting the usb devices that you have plugged in.
This software is quite useful for me, who as soon as I started taking my MacBook to work noticed that I had to manually change my network to get access to through the proxy at work. I don’t generally have any particular USB device plugged in to my computer at any specific location so I use the IP rules to change location. I haven’t had a problem with it at all. I had to leave the IP for work quite general (two numbers long e.g. 128.238.*) because it’s a large network, but at home I can use a fairly specific IP.
It’s great and if you run a Mac on different networks I recommend you try it so that Dave can see that it works with different parameters (and to make your network use easier of course).