If your wi-fi’s been flaky since installing Leopard (mine was disappearing every few seconds, forcing me to turn Airport off and back on again each time), this might be the answer:

* Find the AppleAirPort2.kext file in Macintosh HD/System/Library/Extensions and delete it

* Launch Software Update and install (or reinstall) the Login & Keychain update

* Restart

It did the trick on my Powerbook, anyway.