I've long had problems automatically reconnecting to a certain WPA Enterprise wireless network from my MacBook Pro running Mac OS X 10.5.
Despite deleting the network from System Preferences > Network > AirPort > Advanced... > Preferred Networks, deleting its password from the login and System keychains using Applications > Utilities > Keychain Access, and then saving the network and credentials anew, the OS would prompt me to reenter my password for it every time I reconnected to it.
Some digging around on the web suggested that folks having a variety of Mac OS X wireless networking problems can sometimes solve them by deleting various configuration files, including /Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist.
I checked that file, and strangely, it listed a very old password for that wireless network. Turning off AirPort, deleting that file (I made a backup first just in case), and turning AirPort back on has resolved the problem. I can now reconnect to the network repeatedly (including after a reboot) without being prompted for my password.