CG Channel has an interesting interview with Avatar's motion capture producer James Knight. He talks about the motion capture pipeline used, their virtual camera, and also about the problems they encountered.
On Avatar, the motion capture volume was 72 by 32 feet, and we had 102 cameras capturing the data. We had a scaling volume off to the side where we would do range-of-motion work, and there were seven cameras for that, then 95 capturing the live motion-capture data.
Every morning, we’d ‘snap’ each actor in by recording them in the T-pose. Each one had different marker placements on their bodies, since they’re all different sizes and move differently.