OS X has the nice ability to define different "Locations" which can have different network settings (ie. Dial in numbers, DHCP settings, DNS settings, IP Addresses, etc) depending on where you are working. The desired location setting is easily selected from the Apple menu under "Locations"
The default Location as defined by Apple is called "Automatic" and includes settings for all the communication devices on my Powerbook (Modem, IR Port, Ethernet and Airport). However, I seldom need a connection to all 4 devices simultaneously (OS X can handle this and assign an IP address to each connection apparently) and really only use one connection at a time.
If you turn off unused devices, your laptop will power them down thus saving battery life. So creating separate Location profiles makes sense for the Laptop user trying to squeeze some more time out of the battery. To create additional Location Profiles:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021204052141553