Two passes in siggraph08 camera-ready paper

Just now, I heard a message about preparing siggraph08 camera-ready paper.
Interestingly, there are two passes:

1. Before April 21st, the author should submit a "draft" version for a camera-ready paper. Then some feedbacks are presented to the authors.
2. Before April 30th, the author should submit the "final" version as the camera-ready paper.