As far as I understand it right now, Paraguay is the last country in Mercosur that has to endorse Venezuela's accession. They have to formally endorse this through their congress or their senate, and that hasn't happened. I believe Venezuela has observer status within Mercosur, but it is not formally a member until each of the congresses of the other four countries provides its accord.
On October 25th, 2011. See this statement in context.