I have no doubt that for what it is this is a well designed piece of software, but, I'm gong to stick my neck out and say that purely for reasons of scalability a web server written in Ruby just doesn't seem like a sensible choice.
Mongrel was written by Zed in Ruby. Mongrel2 is a new project.
/this note is me sharing what I've learned from zed's tweets, I've not played with the software yet, please correct if wrong.
Also, "does seem like a sensible choice"?
Apart from that, it's completely written in C and uses language-agnostic ZeroMQ to pass "requests" back to the handler than can be written in any language with a ZeroMQ library (pretty much all of them).