And what if a person shows up for U.S. travel without an ESTA after January 12? They're not going to be able to continue travel.
"Once ESTA is mandatory, all nationals or citizens of visa waiver countries who plan to travel to the U.S. for temporary business or pleasure under the visa waiver program will require an approved ESTA prior to boarding a carrier to travel by air or sea to the U.S. under the waiver program. Travelers who have not received ESTA approval by the anticipated mandatory compliance date may be denied boarding, experience delayed processing, or be denied admission at a U.S. port of entry," says USCBP.
An approved ESTA doesn't automatically mean that a person has been cleared for travel to the U.S. The person will still have to meet all other eligibility requirements at the port of entry.
For more questions and answers on ESTAs, visit http://www.cbp.gov/xp/cgov/travel/id_visa/esta/