More data points here:

Last Sunday, I checked in at DCA for DCA-DFW-GRU-GIG that evening. The desk AAgent couldn't believe I was traveling to Brazil without a visa, and she immediately huddled in the back room with two others before they all came out and reported their discovery that Brazil had a visa waiver.

*Much else* about that trip got FUBAR'ed, and I ended up *Tuesday* morning arriving at a GRU passport control desk. There, the officer said, "Where is your visa". I mentioned the waiver, and a lightbulb seemed to light itself over head; I had my stamp and annoying-size paper and was shortly on my way.