Mac OS X 10.4 Tiger added a neat feature for those of us who use Wi-Fi in many locations. It can now display a list of all of your preferred networks - networks you've connected to before and asked to remember after connecting. If multiple networks can be available simultaneously in particular places, you can rearrange the list of networks so that Mac OS X attempts to connect to them in a particular order.
However, many folks who have upgraded to Tiger aren't seeing the Preferred Networks option in the By Default, Join pop-up menu. (To check your system for this anomaly, open the Network preference pane, choose AirPort from the Show pop-up menu, and look in the By Default, Join pop-up menu.) If you had an AirPort network defined in Panther, the Preferred Networks option won't appear if you upgraded to Tiger. Luckily, you can work around the problem by deleting your existing AirPort network configuration and creating a new one. Follow these steps in the Network preference pane:
When you now select your AirPort network from the Show pop-up menu, you'll see that Preferred Networks is an option in the By Default, Join pop-up menu.