I have heard that TAM Airlines allow smal pets in the cabin. TAM flies from MIA to EZE via GRU with a change of aircraft. I had a colleague bringing his Yorkshire Terrier back to Brazil a couple of weeks ago with TAM and that went fine.

By the way, TAM also has very good in-flight service, superior to AA and with sleeper seats in First and Business from MIA to GRU.

Varig used to allow pets in the cabin, but I believe this is no longer the case internationally.