browser-based authentication fails

I have an ASUS laptop with centrino ipw2100 wireless. The wireless
system works at home, but whenever I have to deal with a wireless
network that uses a browser-based authentication system, I make a
wireless connection but don't get the authentication page in any of
my browsers. Of course, the people who run these networks refuse to
help because I run Linux. Any suggestions about what might be going on
or what I can do to diagnose? The firewall and privoxy are off. Using
xnetload, I can see packets are coming in but nothing is going out.

Bob T.