Hmm, ok, lets start
Did you configure your kernel properly? I think you need "CONFIG_USB_USBNET=m" and "CONFIG_USB_ARMLINUX=y" in your .config. Well, then try "modprobe usbnet" as root. Then put your Z into the cradle, and reboot it (I have to do this, dunno if you have to, and I don't know why; just do it, won't hurt). Then check with "ifconfig" (also as root) if you have the interface usb0. From then on, it should be like before. Do
/sbin/ifconfig usb0 192.168.129.1 netmask 255.255.255.0 up
/sbin/route add -host 192.168.129.2 usb0
as root, where you have to replace 192.168.129.2 by the IP of your Zaurus, and 192.168.129.1 by a randomly chosen one from the same network (i.e., the first 3 numbers should be the same). Then try "ping 192.168.129.2" (again, replace what's necessary), if it works... voila If not, let me know what went wrong...
Cheers
Philipp