I don't know if it's in production, but the AI for "The Path of Go" was written in F#:
http://research.microsoft.com/en-us/events/techvista2010/demolist.aspx#ThePathofGo
The Path of Go: A Microsoft Research
Game for Xbox 360
This demo showcases an Xbox 360 game,
based on the game of Go, produced
in-house at Microsoft Research
Cambridge. Go is one of the most
famous board games in East Asia, it
originated in China 4000 years ago.
Behind the deceptive simplicity of the
game hides great complexity. It only
takes minutes to learn, but it takes a
lifetime to master. Although computers
have surpassed human skills at Chess,
implementing a competitive AI for Go
remains a research challenge. The game
is powered by three technologies
developed at Microsoft Research
Cambridge: an AI capable of playing
Go, the F# language, and TrueSkill™ to
match online players. The AI is
implemented in F# and meets the
challenge of running efficiently in
the .net compact framework on Xbox
360. This game places you in a number of visually stunning 3D scenes. It was
fully developed in managed code using
the XNA environment.
(Someone else mentioned "TrueSkill" already.)
