So, the AirPort slot is *PCMCIA*, or close to it anyway. Note that this is *not* CardBus. PCMCIA is essentially a miniaturized hot-pluggable superset of the old 16 bit ISA slot, while CardBus is a mostly-complete version of 32 bit parallel PCI. (Not PCI-e, that didn't exist when the Pismo was made. CardBus slots are backwards compatible with PCMCIA because the CardBus controller is able to sense what the card is and operate in a legacy mode. The reverse is not true: CardBus cards don't work in PCMCIA slots; they're keyed differently, and if you force one to fit you can damage both the card and the machine.)
Airport cards will (allegedly) work in a regular PCMCIA slot (you'll need to add an antenna, of course), so you'd think in theory that perhaps you *might* be able to plug a PCMCIA ATA card (flash or spinning disk) into the Airport slot, but I wouldn't bet on it. The Developer Note for the Pismo says the AirPort slot isn't just another connector on the PCI1211 CardBus bridge IC. (Which would seem to have been the "easy" way to engineer it, but whatever.) Instead the "KeyLargo" I/O chip, which contains the IDE controller, USB hub, and serial (IR/Modem) ports, hosts several "low-speed" busses to communicate with the power controller, sound chip, and the AirPort card. So you've got two problems:
A: The documentation describes the slot as being a subset of PCMCIA. That may well mean that some signals necessary to run an ATA device are missing. (As MacJunky notes people have been able to make wireless cards not labeled "AirPort" work in there, but in all cases they're Orinoco/Prism cards that use the same chipset as AirPort and thus presumably have the same interfacing requirements.) And:
B: From a software standpoint, since the slot is dedicated to an AirPort card and isn't associated with the CardBus bridge, even *if* there's enough PCMCIA support there electrically to drive a storage device it probably lacks the auto-detection support necessary to recognize anything *but* an AirPort card. (There certainly wouldn't have been any reason for Apple to write any OS/Firmware support to look for a storage device in that slot.) So, again, even if that slot can work as sort of general-purpose psuedo-ISA slot you'll have to write your own driver for anything you put in it, the OS drivers won't know to look there for devices they might otherwise recognize if they were in the Cardbus slot.
If you *really* wanted to try it and see if it "just works" (at which I'd be really surprised) your best bet would probably be a PCMCIA ATA device, or a PCMCIA->CompactFlash adapter. But I certainly won't take responsibility if trying it lets the magic smoke out of your Pismo.
(I'm actually sort of stunned reading the technote that it *isn't* just another hidden slot connected to the CardBus controller, but looking up the datasheet for the controller they used verifies that it's a model that only supports a single slot. So the Technote ain't lying.)
Airport cards will (allegedly) work in a regular PCMCIA slot (you'll need to add an antenna, of course), so you'd think in theory that perhaps you *might* be able to plug a PCMCIA ATA card (flash or spinning disk) into the Airport slot, but I wouldn't bet on it. The Developer Note for the Pismo says the AirPort slot isn't just another connector on the PCI1211 CardBus bridge IC. (Which would seem to have been the "easy" way to engineer it, but whatever.) Instead the "KeyLargo" I/O chip, which contains the IDE controller, USB hub, and serial (IR/Modem) ports, hosts several "low-speed" busses to communicate with the power controller, sound chip, and the AirPort card. So you've got two problems:
A: The documentation describes the slot as being a subset of PCMCIA. That may well mean that some signals necessary to run an ATA device are missing. (As MacJunky notes people have been able to make wireless cards not labeled "AirPort" work in there, but in all cases they're Orinoco/Prism cards that use the same chipset as AirPort and thus presumably have the same interfacing requirements.) And:
B: From a software standpoint, since the slot is dedicated to an AirPort card and isn't associated with the CardBus bridge, even *if* there's enough PCMCIA support there electrically to drive a storage device it probably lacks the auto-detection support necessary to recognize anything *but* an AirPort card. (There certainly wouldn't have been any reason for Apple to write any OS/Firmware support to look for a storage device in that slot.) So, again, even if that slot can work as sort of general-purpose psuedo-ISA slot you'll have to write your own driver for anything you put in it, the OS drivers won't know to look there for devices they might otherwise recognize if they were in the Cardbus slot.
If you *really* wanted to try it and see if it "just works" (at which I'd be really surprised) your best bet would probably be a PCMCIA ATA device, or a PCMCIA->CompactFlash adapter. But I certainly won't take responsibility if trying it lets the magic smoke out of your Pismo.
(I'm actually sort of stunned reading the technote that it *isn't* just another hidden slot connected to the CardBus controller, but looking up the datasheet for the controller they used verifies that it's a model that only supports a single slot. So the Technote ain't lying.)