One of the best presentations I saw this week at OSCON wasn't even part of the official O'Reilly
event. It was Ola Bini's presentation at FOSCON on JRuby. JRuby is 100% ready for prime time, and JRuby on Rails is more scalable than Ruby on Rails itself. The typical Rails installation tightly couples Rails instances to Mongrel instances in a cluster - they're essentially the same thing (although this can change if you use multi-threaded Mongrel instances with Swiftiply). A JRuby on Rails installation adds new JRoR instances in new JVMs, or the same JVM, and new servers are decoupled from that.
Definitely check out the slides.