ESTA was planned to be mandatory from 12th January, but wasn't - largely because of the poor publicity on the subject: It was estimated that approx 30% of passengers on the day knew nothing about the new system. Therefore, an ESTA amnesty was set in force i.e. although it was strongly recommended to have completed ESTA, it was not mandatory. The amnesty period ended on 12th March and ESTA completion is now mandatory.
Completion in-flight of the I94W will continue for the foreseeable future - remember that ESTA is neither a Visa nor a permit to enter the USA: It is simply a permission to board an aircraft with a destination in the USA (Electronic System for Travel Authorization).
It is also worth remembering that ESTA completion is only required for people travelling under the VWP (Visa Waiver Program) i.e. those completing the Green I94W forms in-flight. Everybody else (Immigrant & Non-Immigrant Visa Holders, US & Canadian Citizens, US Resident Aliens etc.) does not have to complete ESTA.