There must be
a mounting frame I'm missing, then. The place for the
card to plug in is a small laptopish rectangular socket, about 4mm x
20mm. There's card rails that would align a PCMCIA-sized card to mate
with the motherboard along its long edge, but unlike an Apple Airport
base station, no PCMCIA socket or anything close to resembling it.
I suspect this is what's biting you:
"The AirPort Card Adapter is required to install an AirPort Card into any
AirPort-ready, G3-based, slot-load iMac. This adapter was included with
AirPort cards made during the same period as these computers."
http://support.apple.com/kb/HT2367
Wonder if that's like the PCI cards that accept PCMCIA wifi cards?
Gordon