Hi, I realize that your post is nearly two months old, but I just got myself a Zaurus sl5500 off eBay too and was wondering whether you finally got it working and how. The OpenZaurus Web site has what looks like a good howto to get started, at
http://openzaurus.org/wordpress/howto/usb-networking/.
It made me realize my problem was with the kernel - the usbnet driver's defective in 2.4.29/30. So, within a couple of days I'm biting the bullet and moving on to 2.6.13.4 (shudder). Anyway, if you (or anyone else) has got relevant pointers to share, I'm all ears.
All the best