Michael Shifter has an excellent article [PDF] in Current History about Latin America's new integration efforts. A few comments.
1) Whether or not you think these newer institutions like UNASUR and CELAC are relevant (my take), the discussion about these new institutions is a hot topic across the hemisphere. There is certainly an interest and a desire across the ideological spectrum to see something newer and better than what currently exists.
2) I think the Pacific Alliance among Mexico, Colombia, Peru and Chile has potential, though like all the other institutions, could stumble as it hits the hard questions about sovereignty. If those countries can achieve additional economic integration, they would be a real force in the hemisphere.
3) Brazil is boosting UNASUR and they see it as their best effort to influence hemispheric affairs in a multilateral way. However, the same way that the OAS eventually ended up being far less dominated by the US, if UNASUR is successful, Brazil's influence in it will eventually be balanced out by the other countries.
4) None of these new organizations is a threat to the OAS as they can all coexist without competing. It's the OAS's own dysfunction that is a threat to the OAS. The organization needs to reform and needs several governments in the hemisphere, including the US, to push for those reforms. Creating a modernized OAS would be a great topic for the Summit of the Americas.
