I've seen the original airport slot discussed in length, it apparently does not have the bandwidth to support 802.11g, thats why no one has ever (as far as I know) made a successful upgrade to one. The easiest thing to do is just use the PCMCIA slot, but the dumb thing will stick out, I know.... If there is room for the card somewhere in the chassis perhaps you could build a cable to route the pins from the PCMCIA slot to the card?
PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...