There are a lot of brazilian restaurants in Boston, specially located in Cambridge.
I have been in two good ones :

Café Brazil
421 Cambridge Street, Allston 789-5980
I can recommend Mandioca Frita com Linguiça-golden fried cassava root and grilled linguiça sausage for appetizer and Churrasco na Tabua-marinated steak, boneless chicken breast and linguiça sausage with fried potatoes and soutéed onios.

Midwest Grill
1122 Cambridge Street, Cambridge 354-7536
You must try Rodízio-the brazilian barbecue, beef, chicken and pork, all you can eat. They keep bringing you food all time.