Even if you have installed the latest Keychain update, you may be facing this issue.
Some users have determined that an artifact left over from Mac OS X 10.4.x interferes with wireless connections after Mac OS X 10.5 is installed. As explained in
this thread, the issue can be dealt with by finding and removing the AppleAirPort2.kext file from your Extensions folder, located here:
Macintosh HD:System:Library:Extensions
It's a simple fix, but one that you would never find on your own. And, it's pathetic that the installation or migrations scripts fail to remove this troublesome component.