I mean, the mvc for cairngorm and the one in rails don't overlap their functionalities? I'm not sure I understand the need for cairngorm with the rails backend..
There is no need, it's an option, Cairgorm see's the whole Rails as a model. Rails send xml instead of html so no browser-css-details headaches.
You can use rails as a REST layer and put your business logic in flex.
You can use cairgorm as a candy cover over a complex but well tested rails application.
You can find middle of the way solution that suits to you.
We do implement most business logic in rails but use flex/cairgorm to show each user only the right buttons/forms/controls to her task, and to do it in a nice browser-independent way.
Hope this helps you, feel free to ask any detail
Rails is MVC for the server. You still need an architecture for a sufficiently complex client application. We're using PureMVC on the client with a REST (JSON) interface to a Rails server and it's working nicely. PureMVC allows for the client's model to be independent from the client's view components, and makes it easy to update independent view components simultaneously.