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.