I have seen several pages where people have tricked an Orinoco .11b PCMCIA card to work in there. It's a bit of hackery, but it can be done, and it shows up as an Airport card in software. Connecting via USB won't give you much joy - it's USB 1.1 only.

Here's a third party Cardbus card which supports something called "Super G", whatever that is.

http://www.wegenermedia.com/108mbpcard.htm