I found ifplugd last night while reading man pages and install it also. The only difference in the way that you did it and the way I did, I used a static IP address.
/etc/interfaces
# The network interface for the Zaurus
iface usb0 inet static
address 192.168.129.1
pointtopoint 192.168.129.201
netmask 255.255.255.0
When I turn the Z on or place it in the cradle on, the usb0 will come up within a few seconds, but for what ever reason it is using IPv6 so the PC and Z will not communicate. If I bring it up with ifconfig then everything is fine. Havent' figured that one out yet.
Allen
Update; After several reboots it is now using ipv4 so the PC and the Zaurus are able to communicate. Not real sure how I fixed the ipv6 problem. Tried it on a new install of Ubuntu and at 1st had the same problem usb0 was ipv6, then while I was trying to resolve the problem it started showing up as ipv4 so I could comunicate.
?