There are two parallelism approaches in Racket, both of which provide OS threads. The most useful one for web servers is called 'places', and is somewhat like Erlang -- you can only communicate immutable serializable data between different places, as well as OS resources like file handles. I know other people have used places to scale web applications, but I haven't looked at how HN's design would be affected.

If you have questions about using this, please feel free to email me about it.