I've driven by the giant guitars in Gavea & Copa but hadn't really looked into what Rock in Rio entails until this weekend. Turns out it's from September 23rd until October 2nd and will feature the following bands (announced thus far):
September 24: Red Hot Chili Peppers, Snow Patrol, Capital Inicial, NX Zero, Stone Sour.
September 25: Metallica, Slipknot, Motorhead, Coheed and Cambria, Sepultura, Angra.
October 1: Coldplay, Skank.
For more info: Rock in Rio site