Already is! I've submitted talks a few times about my experiences with Rails in an enterprise environment, no luck. Anyway, We run rails since 2006, we got large and became a bank.
Numbers: 5032 models, 42 gems, 16 submodules, lots of C extensions and eventmachine servers running in threads. From our website to ATM servers, it's everything inside a single rails application. We managed to upgrade from 2.3 to 3.2 in 6 months. Already running 4 (took only a few hours). We just completed our migration from Oracle DB to Postgres.