Rails is threadsafe if you turn it on by uncommenting config.threadsafe! in your environment initializer.
Somewhere around 3.0.6 I made a simple jruby rails app and a sinatra app. They were both a thin REST layer over data in mongodb (No AR). I did load testing and the sinatra app ran fine, the rails app would fail on about 7% of requests with seemingly random stack traces. When I reran the tests with config.threadsafe! turned OFF it never failed.
Also, on Bob Lee's OSCON talk he mentioned some unthreadsafe code in AR that he found.