My experience with these public networks is that you have to open a browser to connect to them. Do the following: Turn your Zaurus on and insert your wifi card, don't start the network. Open a browser and go to any page. The Zaurus will ask you if you want to connect to a network, say yes and select a network with SSID:any (you need to have one of those preprepared). At this point it should connect and either take you to the page you selected or to their splash page which may ask you to register. I guess now it depends on which network it is.
Felipe
p.s. My experience is with the Sharp ROM, don't know about others.