I did an on-site this past Monday. They allowed me to code the homework in whatever I wanted. I used Haskell. We code-reviewed it in Haskell, with a guy there who knows Haskell.
I had that happen to me once. A candidate (college hire) chose to code in Haskell, which I had a passing familiarity with. We offered him the job, and he was definitely a strong hire, and we used Java mostly.
I also got a homework which I could implement in whatever I wanted and knowing they are a Clojure company… I chose OCaml. We did the review of the code, explaining the interviewer enough of ML to make him being able to understand the code and why I solved things this way. Was a fun day, would recommend it.
Follow-up: someone saw the above comment, asked who I was, and it turned out my middle-school friend worked at the company. Interviewing somewhere new now.
They use Scala and Clojure on a regular basis.
God I hope they offer me that job.