As always, there is no concrete information about when the update will drop, only this vague announcement from Rovio:
Angry Birds Rio Carnival Update Coming Soon!
Looks like the Rovio programmers have been watching the movie Up! but decided to replace the boy scout and cranky old man with a monkey. Ohhh, we're trying to pretend we're excited about this whole thang but really we're just wondering how long this whole Angry Birds hype will last. Okay, maybe we are the cranky old man from Up! Just replace us with a monkey. We do love you, Rovio, though. Really, we do.