I have authenticated using Netfront on the Zaurus to variois free and non-free networks - T-Mobile, Bryant Part Free Wifi service (NYC), Downtown Alliance in Downtown New York etc.
No problems at all.
All you do is firstly connect to the service (setup a Network setting for the said network with Essid, or 'free').
Then laucch the browser and just go to any site (I set google as my load page) - and it will automatically redirect you to the authentication page of the network you are connecting to. Do the necessary authentication and you are set to go...
After the authentication, even email, FTP and all other internet enabled apps start working.
Note : for email sending, you have to setup a SMTP server seperately - thats a whole different topic though.