Assuming you used the values on the web page you mentioned, I think you need to add routes for the subnet (192.168.129.0). If you use a firewall, there could also be problem there. I use guarddog to manage my firewall, and I remember having to configure it to permit ping traffic from and to the Z's subnet (192.168.129.0 for me). If you're using regular Ubuntu (i.e. not Kubuntu), I'm not sure what the GUI program is to configure firewall, and my kung-fu isn't strong enough to recommend changes to ip[chains|tables].
FWIW, I use /etc/network/interfaces to add and remove routes, and (with OZ) it works for me w/o needing to install the /etc/network/ifup.d script that Dreadlocks listed below. IIRC, that script was originally on the docs.zaurus.com site, which was run by Sharp. I'm not sure if its specific to Sharp ROM's. . . All I generally need to do is the following:
1) modprobe usbnet
2) plug in the Z
3) ifup usb0
I'll post my /etc/network/interfaces tonight when I get home.