From:
http://www.visitusa.org.uk/visitors/esta.aspxquote:Airline Acceptance of ESTA
Online completion is mandatory for all Visa Waiver Programme travellers to the USA provided that the airline used is capable of receiving and validating ESTA data. If the airline does not have this capability, then the old manual I-94W form will continue to be used and which will be distributed to passengers by that airline, normally either at check in or in flight.
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Don't know the US destination address or flight details yet?
ESTA applications can still be made without a full address and flights. In the short term flight details are not required and a generic address only will suffice. E.g. City and State. Applicants should register full address & flight details once known.
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Repeat Visits? Change of Itinerary?
Updating ESTA data for repeat visits within the 2 year ESTA duration is recommended but is not mandatory. I.e. Upon entry into the USA, the Customs and Border Protection Officer may ask additional questions if a traveller's current itinerary differs from information submitted in the ESTA application. Updating ESTA with new arrival and first night's stay data could save time at the point of entry and result in fewer questions.