I just returned from two weeks in Mexico. I cleared immigration/customs in Houston where for the first time in many years of regular international travel I was led with all other returning US citizens, to new kiosks which scan the passport, ask for confirmation of information and take a photograph. The printed receipt is then taken to the same immigration desk one usually passes for the standard check and where the printed receipt is taken.
DHS ( http://www.immihelp.com/visas/usvisit.html and http://www.dhs.gov/how-do-i/visit-united-states ) report that the fingerprint/photo process applies to international visitors and green card holders but “US-VISIT does not apply to U.S. citizens”. When the US-VISIT program was created in March, 2013 there was an outcry against collecting biometric data on US Citizens. This resulted in the exclusion cited above.
It appears that DHS has decided to surreptitiously violate this policy. Along with the digital photograph, is facial recognition and/or retinal scanning being used? Can we know? Likely only through patient FOIA requests.