Okay, can you then please try one or two alternative, compatible chat clients, like Psi or Pidgin? Google Talk is (well, should be) just another Jabber/XMPP client, and we're doing most of our tests with Psi and Pidgin (because they are available for Linux too).
Can you take a look at the http proxy and packet filter logs too? There should be some evidence why google talk fails.
