I looked at this last week over at the Standard.
Today Reuters looks at Brazil's stark choice: make an enemy of Hugo Chavez, or make Mercosur largely irrelevant.
The Economist also has a good look at Mercosur's challenges:
Mercosur has plenty of other problems. They start with the big difference in size and government policies among members. Under Mr Kirchner, Argentina's priority has been to protect inefficient but labour-intensive industries as it recovers from socio-economic collapse in 2001-02. Smaller Uruguay and Paraguay complain that the group has done little for them.
Brazil has made periodic attempts to “relaunch” Mercosur. It is the main contributor to a $100m development fund that is seen as a modest copy of the European Union's regional funds. Its diplomats say they want fewer trade restrictions and, at last, a common customs code (at present each country collects its own customs revenue, and tariffs are levied on goods from outside the block each time they cross a border). But in Asunción implementation of this was postponed yet again.