If you hold down the option key while selecting a secure wireless network (WPA only) from the AirPort menu bar icon, the system brings up a different dialog box. Instead of asking for the ASCII password, it lets you enter a hexadecimal password (noting you need exactly 64 characters) instead. [robg adds: This worked for me in 10.5, but I'm not sure if if works in 10.4 as well, as my 10.4 test system isn't AirPort equipped.]

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