Thank you very much for your suggestions and interest everyone. The  
pointers to:
	* Ruby Inline: http://www.zenspider.com/ZSS/Products/RubyInline/
and
	* Ruby DL: http://ttsky.net/ruby/ruby-dl.html

are exactly what I was looking for, so thank you! I'll get back to  
you with any exciting a amazingly fast code :)

As an aside, I noticed that the benchmarks at RubyInline are using a  
factorial computation. The factorial is only for 5 though. I would  
guess that for such a small factorial there could be reasonably high  
overhead for the c code's calling, so it doesn't really get to shine  
against the Ruby implementation.