Why won't my MacBook Pro connect to Wi-Fi networks that use a sign-in page (captive portal)?
I had this problem connecting to the local public library system network. The problem seems to be a result of my having specified DNS servers ((i.e. OpenDNS, Google, etc.)) in my Network preferences. The solution was to create a network location called "No DNS" which doesn't have any DNS servers defined and use that network location when I need to log into these networks.