Later: Matylda Czarnecka, from siggraph.org, asked me to post this: Many apologies for the confusion. We experienced a technical problem today which caused the review form to appear behind a login screen. We just resolved the problem and the link you received in the e-mail should now function correctly. The passport field appears on the general sign-up for new SIGGRAPH accounts and is NOT a required field. We certainly never intended to require references to create a SIGGRAPH account just to review an applicant, and we apologize for the confusion! Thank you for your patience. Please try the link again and let me know if you have any further questions or problems.
Later still: Mk Haley, also of SIGGRAPH, sent the below. Ironic that I spent half an hour in my Software Architecture course last week ranting about the pervasive loss of privacy in the wired society... This optional field was added to the electronic submission system in response to the massive number of visa letters that SIGGRAPH is asked to produce each year for our foreign student volunteers and attendees. For many, they have to produce an official letter from the conference as part of a visa application to get into the country to attend. By compiling this information on the front-end, we have helped streamline that process for our international attendees. If that is not an issue for any participant, or they would rather handle the data transfer in another way, they are welcome to do so. For any questions related to ACM SIGGRAPH, The SIGGRAPH 2007 Conference, or the Student Volunteer Program, please tap us directly, we are delighted to provide the information you need to participate. ACM SIGGRAPH Director for Communications - Mk Haley firstname.lastname@example.org