The U.S. does not presently have an income tax treaty with Brazil. Due to differences in opinion between the two countries as to the proper scope and provisions of such a treaty, it is unknown if one will ever be forthcoming.
Nonetheless, the U.S. and Brazil have now entered into an Exchange of Information Agreement. Such agreements provide a mechanism for each country to assist the other in tax investigations and administration.
The Agreement only provides for the delivery of tax information upon request by one country of the other. It does not have any provisions for spontaneous exchange (that is, where one country has an obligation to deliver tax information to the other country because the first country simply thinks it would be of interest to the other country.
http://www.ustreas.gov/press/releases/reports/braziltiea2007.pdf for the text of the Agreement.