Problem most people describes after installing Leopard on Mac Pro stations and some iMacs Airport works but couldn't find any WiFi networks.
Symptoms: Airpot indicator on the tool bar Grey, no networks found.
Aiport device Broadcom 43xxxx
Solution: Install Mac OS update "Login & keychain" from official site. After reboot problem will be fixed.
Other possible solutions for this particular problem: In case you upgraded from previous tiger version in /Library/Extensions/Airport remove Airport.kext (part from Tiger version of Airport driver, not needed in Leopard). Try boot in safe mode (hold Shift during boot) and restart.