AB Rio has actually been free every day on the Amazon App Store since the launch. I think today is the last day you will be able to get the paid version for free. It was the only free paid app on the launch of the Amazon App Store and every day since there has been a different free paid app in addition to the paid version of AB Rio. I'm guessing that since they're making AB Rio the only free paid app again today it's the last day Rio will be free (for the add free version).
I'm loving the paid apps Amazon has been giving away! With a second child on the way there is no money in the gaming budget so it has been great to get some cool apps like Angry Birds Rio and Fruit Ninja. I also like the layout and design of the Amazon app store. It's nice to see two columns of apps and I like the presentation of the sample images and videos better than the way it's done on the Market.