Hi Ryan - There are numerous TechHuians who are into functional programming. Tim Dysinger mentioned in this thread that he is doing 100% of his programming in functional languages. Ryan Kanno and Pierre Omidyar are using Erlang at Peer News (the company behind Civil Beat.) I remember hearing a Lisp talk by Kaveh Kardan a few years ago at a Honolulu Coders meetup. There are 11 members in the Erlang group.
Being a language geek I've played around with Erlang, Clojure and F#, but I haven't used them in production systems.
In my former startup we used OCaml quite a bit. Being a functional language, if you are not used to, it takes a bit of getting used to. Though the conciseness and language constructs like closures gave us a good boost in production speed.
The problem we found with OCaml was the lack of good libraries for game-development related stuff like 3D rendering. So, I would advice you to take a look into how many good ocaml libraries that available for you specific kind of project. We ended up building our own interop layer between Ogre3D (which is a C++ library) and OCaml. An undertaking which ended took far more time than we had thought it would.
If you are into .Net, then F# is adapted from OCaml, and you have all the .Net out there available for you.
I have written actual software in Scheme. One of my favorite technical manuals is the manual for the "Scheme Shell", written (the language and the manual) by Olin Shivers. Never used Haskell & Co., but like other language geeks I enjoy reading/thinking about them. I seem to have lots of books either about functional languages or which use functional languages to illustrate some concept or other.
Also, XSLT is a functional language. XSLT 1.0 left a nasty taste in many folks' mouths (mine included) but version 2.0 of the language is a huge step forward and is actually kinda fun.