I've always wondered about the potential for using the slot for something else also.
It will depend highly on what kind of controller Apple used for it...is it actually anything close to a real PCMCIA controller, or is it just a hack that does just enough to handle a few extremely particular wifi cards?
(And my guess is the only reason the orinoco works, almost out of the box, is that it is very close hardware to the actual apple airport cards)
And, of course, this may vary from model to model...so, for example, how close it is to a real PCMCIA slot may be quite different in early vs later machines.
It'd be interesting to look at the OF device tree for what it looks like...and running linux, what does the port show up as a pci device? (Take any apple operating system possible obfuscation out of the equation).