The SENSEable City Lab @ MIT has an interesting project about using visualization to analyze how footbol players move on the field during a match:

Information about the movement of soccer players on the field during a match can be useful for strategic and physiologic analysis, directed at improving the performance of the players and that of the team.

There are some interesting challenges like the fact that "the sensing area is large compared to the moving actors (players)" and that a strong occlusion is always present due to the paths that are most frequantly traced by the players.

There are a number of other visualizations tracking people moving in a space and, interestingly enough, they always have the same recognizable shape (you can say ah ah, this is people!) and share similar challenges. Notably, Katy Broner's work on Visualizing 3D Virtual Worlds and Their Users (some images here)