The following, although not strictly on thread may be useful.
Nevertheless, on our next visit, I think I'll fill in the I-94W until demonstrated as no longer required.
DHS: Frequently Asked Questions: Electronic System for Travel Authorization (ESTA)
Embassy of the U.S. London: DHS (Immigration)
Q: If people have received ESTA approval, do they also need to fill out an I-94W?
A: DHS has been coordinating with commercial air and vessel carriers on the development and implementation of messaging capabilities for passenger data transmissions that will enable DHS to provide the carriers with messages pertaining to a passenger's boarding status. A prospective VWP traveler’s ESTA status is a component of a passenger’s boarding status that has been introduced into the plans for implementing messaging capabilities between DHS and the carriers. The implementation of the ESTA program will allow DHS to eventually eliminate the requirement that VWP travelers complete an I-94W prior to being admitted to the United States.
After Jan. 12, 2009, a VWP traveler with a valid ESTA will not be required to complete the paper Form I-94W when arriving on a carrier that is capable of receiving and validating messages pertaining to the traveler’s ESTA status as part of the traveler’s boarding status. Until ESTA is mandatory, however, all VWP travelers applying for admission at a U.S. port of entry, including those VWP travelers who possess a voluntary ESTA authorization, must still present the Form I-94W to U.S. Customs and Border Protection.