Non-secure Jabber/Google Talk connection in Messages
econ-mit posted the answer here:  https://discussions.apple.com/thread/3745358?start=0&amp;tstart=0

Messages > Preferences... > Accounts (select Gmail Google Talk) > Server Settings > uncheck "Automatically find server and port" > click SSL

When I did that, the Server field changed from gmail.com to talk.google.com and it immediately worked.