I know that the WiFi card in my iBook cannot be replaced with an aftermarket one. There's simply not enough space. It's about two-thirds the length of a normal PCMCIA card. And it was awkward enough to attach the antenna when it was designed to fit that particular card.
Apple also claimed that it wasn't really a PCMCIA slot and that normal cards wouldn't work anyway. I never tested that out, though.
Anyway, I wouldn't be surprised if the Mac Mini was the same way.
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Bitt Faulk