π or Pi is a 1998 psychological thriller, directed by Darren Aronofsky, centering around a number theorist and the mathematical concept of pi. I was fascinated by the film, first by the character of Max, the number theorist, and then by wondering exactly how much of what happens takes place in his mind and how much takes place in some form in an objective reality. It begs for a second viewing.
The game of Go features prominently in the movie.
Strangely enough, it's currently available online through googlevideo:
9/10/2009: Weird Movies has a review.