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.
In which case I am remiss, although at time if writing it says here http://en.wikipedia.org/wiki/Mongrel_(web_server) "Mongrel is an open-source HTTP library and web server written in Ruby by Zed Shaw.".
Mongrel and Mongrel2 are two very different projects that have very little beyond their name, author and the fact that they both are servers in common. They share no code and have very different designs and goals.
Well Ragel supports Ruby code output as well (it can generate any of C, C++, Objective-C, D, Java and Ruby). But I'd be surprised if Mongrel used that.
Some people do that. It actually was used in jruby, and I think a couple projects generated the raw ruby rather than the C version. Uhm, I want to say Rubinius?
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).