Well, not anymore. At least not with Rails running on the JVM using JRuby. Look at this post, where Charles Nutter discusses deployment of Rails on a Glassfish application server as a single war.
Apparently, simply by instantiating multiple JRuby runtimes inside the war file, we are able to circumvent the problem of thread-safety in the Rails framework. Just have to make sure there is only one active thread in each JRuby runtime.