So I updated the firmware in my Airport Base Station, and then spent the entire day on the phone with Apple trying to diagnose the problem with my brand-new 21.5" iMac's sudden lack of connectivity to my Airport Extreme Base Station. My 13" Macbook 13" connected, as did my extremely old iMac G4 and my iPod Touch 4G, so we thought the problem must be with the new iMac. After reinstalling the os from the install discs, we determined it must be the Airport card, so I planned on losing the machine to the Apple store for however long it was going to take to fix it.
In desperation, though, I tried reverting the firmware in the Airport from 7.5.2 to 7.4.2. Guess what? All better. Shame on Apple for not suggesting this.
You can easily figure out how to do this by cruising the forums either here or Apple.

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