When you make changes to your preferred networks in AirPort preferences on a computer with Tiger, please note that the settings will affect all of your Locations.
Before Tiger, you were limited to one AirPort TCP/IP configuration per Location, which forced some people to create additional Locations to meet the configuration requirements of different networks. Tiger lifts that limitation, so you can create extra port configurations in one Location, and Tiger will automatically use the appropriate one. This means you can now go from place to place without touching the Locations menu.
The only people affected by the new global nature of preferred networks are those who are in the presence of several wireless networks at the same time and were using the Locations menu to switch between them. The preferred method for achieving this effect is to simply switch networks in the AirPort menu.
If you don't have this menu in your menu bar, go to the AirPort section of Network preference and check the box for "Show AirPort status in menu bar."