Well back in the days of Growing Up Catholic and Nuns Having Fun we didn't have uns who looked like Streep--or if they did we couldn't tell--you know, because of the WHIMPLE.
Anyway, Newsday has an interesting piece about Streep's inspiration for her Oscar nominated role in Doubt which is a good read. Her portrayal is firmly rooted in 1960s Catholic school and a little bit of Rudy Giuliani?