Currently Gabble's 1-1 text channels only unbind when the other client says <gone/> (or you close the channel), or you receive a message from another resource. This covers two of the three cases in <http://xmpp.org/extensions/xep-0296.html#general-unlock>.
Gabble should unlock when it receives presence from the contact. It should also probably unlock after some period of inactivity.
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