From the Washington Post... (Dead Link)
The 9/11 Commission saw the visa system as a major vulnerability and recommended completing a biometric system that would log immigrants out as they left the U.S. This program, however, was never fully implemented.I think a lot of people would be surprised to learn that a comprehensive automated system for recording the departure of visitors wasn't in place at US entry and departure points. It's the kind of thing people just assume happens as a matter of course.
Biometrics can help to automate and audit this process at a reasonable cost.
See Also:
Multiple ID's But Only One Face
How DMV Face Rec Can Prevent Identity Theft
