Hi again zaurusen,
The fact that you now have usb0 coming up as 192.168.129.1 is half the battle. I know that you said your SL-5500 was set to 192.168.129.201/255.255.255.0.
How did you set the IP of your Zaurus? In Linux syncing over the cradle is done via the usbnet driver. This kernel module (driver) allows for TCP/IP to run over the USB. Hence, in the "Settings" tab of your Zaurus, under the "PC Link" application the Hostname field should be set to "zaurus" (without the quotes), the USB IP field should be set to "192.168.129.201" (without the quotes), and the Connection drop down list should be selected on "USB - TCP/IP (advanced)" (without the quotes). Hence, if any of that info is not set correctly please set it.
The Zaurus is NOT like most PDAs and rightfully neither is it's cradle. The Linux Desktop does not recognize the Cradle itself as a device. Hence, if you were to plug in the cradle while the box was on while running "tail -f /var/log/messages" you would not see anything happen. Also, if you plug your Zaurus (while it is turned off) into the cradle you will not see anything happen. However, when you then turn on your Zaurus you should see messages about it recognizing the Zaurus device and it loading the usbnet kernel module (driver). As you can see, the Linux Desktop is actually detecting the Zaurus as the device and not the cradle.
After the above setup has been done. What is the output of the following commands?
Desktop (with Zaurus in connected cradle):
ifconfig
ping 192.168.129.201
Zaurus (in connected cradle, use the terminal application):
ifconfig
ping 192.168.129.1
Given the above info I will be able to help you better.