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.