Would it be possible and how hard would it be to create an external PCI enclosure (connecting via USB/Firewire). This would allow for us laptop users to still use full size desktop PCI cards (like the 0404 or 1212, amoung others).

I take it the only reason this hasn't happend is because it's not possible? Or perhaps it has? I know the PCMCIA sockets on laptops are the PCI equivalents of desktops...but the PCMCIA 'PCI' incarnations are few and far between and are generally a lot more expensive.

A good example of where this has happend before is with 3.5" external enclosures (which I use to connect a 3.5" 250GB HDD to my laptop).

I think this even exists, but is strictly intended for development purposes. You'd have to write the device drivers yourself. Besides, a PCI bus connected via USB or FW would not be treated as such by the OS, thus you couldn't use the normal drivers for the cards. The only chance would be using a PCCard slot, as the Cardbus controller basically is a PCI-PCI bridge. You could attach another PCI-PCI bridge there. Even then, it's going to be very critical RF wise, as PCI is just not intended for any greater trace length, let alone some kind of ribbon cabling. (Besides it wouldn't be easy to get so many signal lines out. The maximum I could imagine as being possible is some kind of PCB sticking out of the PCCard slot which then plugs into a riser card of sorts that can slide up and down in an external case to accomodate different heights. It's unlikely to look pretty, may not be easy to get stable mechanically and will probably leave you with a potentially disturbing thingway right next to the notebook.) In addition to that, you'll also need +/-12V supplies (+3.3V and +5V can be had from the slot, though I don't know whether 3.3V cards actually have access to 5V supply), which adds an external power supply. And even if you have the whole stuff up and running you might still run into trouble with crappy Cardbus controllers that have already caused problems with other PCCard sound interfaces.