I believe Flex and Rails are still being used in production somewhere. Here is an article
focusing on a Rails app which moved to a Flex front end (they eventually ditched RoR).
Personally, I'm more interested in using Stephen's RocketAMF gem
with Sinatra and Mongoid to create a more service centric backend with a Flash/Flex client side frontend. After working with Flash/Flex and Rails, I believe it is a bit overkill (rendering templates, globbing hundreds of helper methods, ect) depending on your needs. If all you want is a custom widget that speaks AMF with a Rails backend then rubyamf is your bet. In my case, I want a full fledged desktop/web app without much/any templating. Eventually I'll test this idea out and post a demo on github.