If you've installed the software from the original Sharp ROM, you need to go to the Zaurus Manager and set the connection type from USB - I/O to USB - TCP/IP. Then make sure that the address in the "Host IP" box matches what you set for ubs0 in the Network settings app. FYI, you have to have the Z in the cradle when you make that change, and I usually have to take it out and then re-insert it into the cradle. The usb0 connection usually comes up no problem then.
You should be able to connect through Qtopia Desktop then. I have to say, though, that I've not had much luck doing a complete sync with it. Calendar crashed twice on my yesterday, and I've never gotten the address book to go through without a bazillion security warnings. But then, I'm using the Outlook plugins. . . if you're just sync'ing with Qtopia Desktop, maybe your experience will be better.