<p>I know about the "cooperative" threading of ruby using <a href="http://en.wikipedia.org/wiki/Green_threads" rel="noreferrer">green threads</a>. How can I create real "OS-level" threads in my application in order to make use of multiple cpu cores for processing?</p>

Ruby 1.8 only has green threads, there is no way to create a real "OS-level" thread.  But, ruby 1.9 will have a new feature called fibers, which will allow you to create actual OS-level threads.  Unfortunately, Ruby 1.9 is still in beta, it is scheduled to be stable in a couple of months.
Another alternative is to use JRuby.  JRuby implements threads as OS-level theads, there are no "green threads" in it.  The latest version of JRuby is 1.1.4 and is equivalent to Ruby 1.8