RoR for dynamic HTML apps used to be the coolest thing but now the really new/cool stuff is written as a JSON server talking to AJAX and mobile clients. In this world node.js, clojure and erlang (maybe) are cooler than RoR.
In the non-startup enterprises (I work with those), the work is usually to glue things together. Call a legacy web-service here, run a worker there, put things back into a legacy CRM etc. Ruby is perfect for that.
JRuby is even better, actually, and I wouldn't be surprised to see Heroku support it later on.