Hmm, has the code been done already?

We've got a sharp zaurus here. It's a StrongARM PDA running linux that does Ethernet over USB.

On my desktop i run either a patched CDCEther driver that gives me an eth1 interface, or with newer zaurus software you use the usbdnet driver which give me a usb0 ethernet interface.
(Not sure if the usbdnet driver is in the stock kernel tree, but mandrake 8.2 ships with it)

Now the source for the linux kernel on the zaurus is at http://more.sbc.co.jp/slj/source/source_dl.asp (may require registration)
Perhaps you can just grab the ethernet over usb driver from there?