In interpreting a 1996 law in the light of the post-September 11 legislative environment, the U.S. Government is making fingerprints and face scans the currency of travel. Without disclosing this information you will not be able to travel. And the system will be extended to other biometrics. This process will wrongly identify individuals as being 'of interest'.
US-VISIT is not a system for countering terrorists; it is an immigration control system that will be used for an increasing number of purposes, with fingerprints being traded around the world, particularly as other countries take the torch from the Americans and establish similar systems.
This information will be used and abused, and even if you have nothing to hide, you can find yourself in prison or in shackles on a return flight to your home.
US-VISIT is the gold standard for privacy invasion.