[Freaky]

It's not the webserver, it's Rails; prior to 2.2 it used a Big Giant Lock around the dispatcher, so it would serialize requests.

Of course, even without that Ruby itself will only use a single CPU, since the interpreter itself has a Big Giant Lock, but it can still use threads to multiplex requests and avoid wasting time waiting on every IO.

JRuby allows for proper concurrent multithreaded request handling; I'm surprised it's not a more popular deployment option.