A note to folks: this is a "pre-N" router - that means that it's using what Apple believes to be the right implementation of the spec as they read it. 802.11n is *not* ratified as a formalized spec by IEEE yet and as such different router's code may work differently based on how the programmers read the draft of the standard. Most assuredly your 802.11n iMac/whatever client works with it, it should be obvious why; this does not mean, though, that non-Apple gear will always work with it.
Personally if I had one, the first debugging step I would do is disable any and all 'N' functionality on the Airport Extreme device and go from there.
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IEEE 802.11n - Wikipedia, the free encyclopedia
802.11 non-standard equipment - Wikipedia, the free encyclopedia